Chroma is great - if you use his abilities right (I recommend ICE energy) you can get massive, massive armour values. His 4th ability is useful in defence missions but the real value is as a solo tank in high level missions.
Don't diss Hydroid or Oberon. Oberon is a really difficult frame to learn to use, but is extremely flexible. Hydroid is not great, but is a lot of fun, especially with the Pilfering Swarm augment.