When I first started dis Warframe game I wasn't interested in anything prime... I thought learning how to mod would be enough... Then I seen Equinox Prime and Life was never the same. Which was your 1st prime Warframe?
My first frame after excalibur was A pre-rework Vauban that I leveled to 22 before realizing he was bad and then trashing him.
Then a while later I got Oberon. I actually hadn't at all bothered with prime stuff up until around MR10. thats when I got Mesa prime. though my first built prime item was the akstiletto prime around MR10. I now have 3 prime frames but am still a harrow main.
My first prime was Frost because they were giving it away on twitch to whoever had twitch prime together with vectis and scindo prime I believe. But my first prime that I legitimately got and made a proper build for was Valkyr. She was also the first I made full umbral, with all 3 umbral mods.