I have ranked the warframes based on difficulty of their acquisition, as of PC Hotfix 27.5.3.
1. Harrow
Harrow's main blueprint is awarded upon completion of the Chains of Harrow quest. Its an Ok quest, but so much time invested just for a BP
Chassis - Void Fissure Enemies (if you are eligible to farm his other parts, you might have tons of this already)
Neuroptics - Rotation C vault on Pago, Kuva Fortress (Requires War Within quest completion, and its not a spy mission with adequate tenno-traffic)
Systems - Defection missions on Rotations B and C. (Arguabily the most hated mission type in the game)
2. Nidus
Nidus' main blueprint is acquired from The Glast Gambit quest. Nidus' component blueprints are acquired from Rotation C of Infested Salvage (Oestrus, Eris). Its a twisted quest and the salvage mission is almost never played except to farm Nidus. It takes a good 20 minutes for a rotation C and its not guaranteed to get atleast 1 component. So, you might end up grinding hours, most probably alone, for nothing.
3. Khora
Khora's main and component blueprints are acquired from Sanctuary Onslaught missions. Every two stages is a rotation, following the standard A-A-B-C pattern. Thats 8 rounds of SO, that lasts for around 30 mins. And the drop chance is low. So, hours of farming for nothing.
4. Mesa
Mesa's component blueprints are acquired from Mutalist Alad V (Mutalist Alad V Assassination, Eris). To fight him, players need to craft the Mutalist Alad V Assassinate Key with the blueprint awarded from completing the Patient Zero quest. players have to acquire Mutalist Alad V Nav Coordinates as a battle pay reward from Infested Invasions or Orokin Derelict Survival/Defense reward or resource caches from Hive Sabotage, as well as a blueprint awarded from the Patient Zero quest in order to create the required key. This is where it gets crazy. The infested invasions are not that common, like maybe 1 per 24 hours and the sabotage caches are a hard grind. And its not a guarantee that you'll get all parts from just 3 keys. So depending on your luck, the grind might go on from days to weeks.
5. Grendel
Grendel's component blueprints are awarded by completing certain missions on Europa using Locators, which can be purchased from the Arbitration Honors vendor found in any Relay for 25 Vitus Essence each. Arbitrations are accessible only after 100% completion of star chart and a total of 75 vitus essense is a grind. The locator missions are another pain in the ass. Just pray that no mini bosses spawn during one of those. Stalker spawned in mine and he lasted 6 whole rounds of defense. He also requires PoE resources to craft
6. Octavia
Octavia's main blueprint is acquired upon completion of the Octavia's Anthem quest. Its an interesting one but a bit difficult. Highly recommended to not try this before completing the War Within quest
Chassis - Lua music puzzle. More of a memory game testing your parkour accuracy. Its a 100% chance, so find the room and you have it.
Neuroptics - Rotation C of Orokin Derelict Survival. Might be a hard farm, but the other rewards in the table are Axi relics, so it will be worth your time.
Systems - the A Rotation Cache of Lua Crossfire exterminate missions. So ideally if you just keep doing lua crossfire exterminate mission and you find the music room and 1 cache per mission, you might end up having both her systems and chassis in the same mission.
7. Ivara
Ivara's main and component blueprints are acquired from Spy missions. All parts are in Rotation C of their respective reward tables, meaning they require three successful data extractions to be an eligible reward for the mission. You can literally passive farm her. The Lua spy is overrated. YOu can do other mid-tier spy missions instead, as the odds aren't that higher for Lua spy.
8. Trinity
Trinity's component blueprints are acquired from Ambulas (Hades, Pluto). Challenging Ambulas on Hades, Pluto requires 20 Animo Nav Beacons. Farming these beacons is a pain. YOu will need a minimum of 50 to attempt the Ambulas 3 times, assuming you get all 3 parts in those, as one ambulas drops only 5 of it. Besides that the mission itself is heavily time-gated, babysitting a bag of junk for 2.5 mins, lasting a total of around 20 minutes in total. It might take hours farming, only consolation is that if you kill, you get atleast 1 part.
9. Equinox
Tenno must forge both the Equinox Night Aspect and Equinox Day Aspect in the Foundry – each of which requires its respective Neuroptics, Chassis, and Systems – before the Warframe itself can be built. Component and Aspect blueprints are acquired by defeating Tyl Regor at Titania. The mission in itself is not that hard, but it does take around a week just to build all her parts and put her together, without rushing.
10. Titania
Titania's main and component blueprints are obtainable from The Silver Grove quest. Scanning plants is a pain, but its easier if you know where. Also you will have to do the scanning again for mod farming and NW challenges.
11. Baruuk
All of Baruuk's blueprints can only be acquired by purchasing from Little Duck, using Vox Solaris standing. To fully access this Syndicate, the player must complete the Vox Solaris quest and The War Within. After becoming an Old Mate in Solaris United, Eudico will hand out Heists against the Orbs. Acquiring Old Mate will require weeks of playing depending on your MR.
12. Revenant
Revenant's main blueprint is awarded upon completion of the Mask of the Revenant quest, which is unlocked after reaching the rank of "Observer" with The Quills. Revenant's component blueprints are acquired from Bounties. Building at least two different components is required to advance through quest. YOu also need completion of the War Within quest and Saya's Vigil to haev access to the quills.
13. Limbo
Component blueprints are awarded from The Limbo Theorem quest. Quest requires archwing and you can only build 1 component part at a time. So it would take around 5 days to complete the frame.
14. Mirage
Component blueprints are awarded at the end of each mission for the Hidden Messages quest. The previously awarded component must be built to obtain the next components. The quest requires 4-5 days to complete without rushing. The riddles are a bit hard to guess without the help of the wiki.
15. Chroma
Chroma's main blueprint is rewarded upon completion of the quest The New Strange. Component blueprints are awarded by completing Junctions. The tricky part is, his components require fully built components of other warframes - Frost, Ember, Volt and Saryn. Saryn will be tricky since its a draggy farm in itself
16. Atlas
Atlas' blueprint is awarded after completing The Jordas Precept Quest, which unlocks the Jordas Golem Archwing Assassination mission on Eris that drops the remaining component blueprints. The Jordas Precept quest requires farming juggernauts for Pherliac Pods blueprints and components, among which, farming the BP was a pain in the ass before they massively buffed the drop chance from 10% to 90%. Alternatively you can tag along with others doing the Assassination mission, which also requires a good Archwing and a good Archgun for effective
17. Garuda
Garuda's main blueprint will be awarded upon completion of the Vox Solaris quest. Garuda's component blueprints are acquired from Orb Vallis Bounties. She requires Kuva, Toroids and Vallis resources for crafting
18. Hildryn
Hildryn's blueprint can be bought from Little Duck for Reputation upon reaching the rank of Agent with Vox Solaris. Her component blueprints can be acquired as drops from the Exploiter Orb. She requires Lazulite toroids, but you will most probably have enough when you have farmed her component BPs
19. Gara
Gara's main blueprint will be awarded upon completion of the Saya's Vigil quest. Gara's component blueprints are acquired from Plains of Eidolon Bounties. She requires PoE resources for crafting
20. Inaros
Inaros' main and component blueprints are obtainable from the Sands of Inaros quest. The quest blueprint is purchasable from Baro Ki'Teer. The quest is relatively easier, except if you get Manics or denial bursa at the final stage. And the final stage is a bit of a drag.
21. Nekros
Nekros' component blueprints are acquired from Lephantis (Assassination, Orokin Derelict). The key requires Lephantis Nav Coordinates. These Coordinate orbs are found in all missions in the Orokin Derelict tilesets, dropping in addition to the regular Nav Coordinates. The grind can be made easy bu doing vault runs and attempting lephantis when you are bored. The mission itself might be overwhelming for newbies.
22. Saryn
Saryn's component blueprints are acquired from Kela De Thaym (Merrow, Sedna). Challenging Kela De Thaym in Merrow, Sedna requires 25 Judgement Points. Farming Judgement points is relatively easier than most grinds and you get good amounts of Endo on the way to Kela.
23. Gauss
Gauss components drop from Tier C Disruption on Kelpie, Sedna. Each component has a 7.84% drop chance.
24. Ash
Ash's component blueprints are acquired from Manics. The only guaranteed drop zones for Manics are defection missions. It is advisable to farm Ash alongside Harrow systems. If you are very lucky, you'll get all the parts from a single kuva survival mission lasting for 1 hour.
25. Wisp
All components blueprints including the main blueprint are acquired from defeating the Ropalolyst on Jupiter. Requires completion of the Chimera prologue. The mission itself is not bad.
26. Hydroid
Hydroid's component blueprints are acquired from Councilor Vay Hek (Oro, Earth).
27. Ember
Ember's component blueprints are acquired from General Sargas Ruk
28. Nova
Nova's component blueprints are acquired from Raptors (Naamah, Europa). This is the only mission that uses "F" key to drop something. YOu cant throw, you have to get in top and drop. It requires 2-3 runs to get a hang of it and after that its ezpz.
29. Frost
Frost's component blueprints are acquired from Lieutenant Lech Kril & Captain Vor (Exta, Ceres)
30. Excalibur
Starter frame / Excalibur's component blueprints are acquired from Lieutenant Lech Kril (War, Mars).
31. Valkyr
Valkyr's component blueprints are acquired from Alad V (Themisto, Jupiter).
32. Rhino
Rhino's component blueprints are acquired from Jackal (Fossa, Venus).
33. Loki
Loki's component blueprints are acquired from Hyena Pack (Psamathe, Neptune).
34. Mag
Starter frame / Mag's component blueprints are acquired from The Sergeant (Iliad, Phobos).
35. Nyx
Nyx's component blueprints are acquired from Phorid assassination missions during an Infested Invasion. One of the easiest miniboss. All her parts can be farmed in minutes.
36. Oberon
Oberon's component blueprints are acquired from Eximus units. All Eximus units have a 1% chance to drop a component blueprints. Easiest passive farm in game.
37. Vauban
Component Blueprints can be bought from the Nightwave Cred Offerings using 25 creds each, for a total of 75 creds.
38-42. Dojo Frames - Banshee, Nezha, Volt, Wukong, Zephyr