The paladin and the necromancer.
Before we get into the frames there's currently a code for two free weapons (and weapon slots) you can redeem on the warframe website on in-game market (use the button on the bottom right): WF-DEATH-BATTLE
Notsure if that's due to the upcoming Tennocon or thanks to the recent Death Battle involving an Excalibur, but either way its a win for us - use it quick before it expires! On to the frames...
Oberon
Not sure how I got my hands on this paladin class warframe as it was waiting for me upon my return, and I found it to be just ok. No super strong offensive abilities but it can buff and heal allies while being pretty sturdy itself. After completing mastery on it I let it go because I'm usually a lone wolf type player anyway.
Then Wrothak shows up in a prime version to help me on a few missions (those freaking Orb Mother Heists on Venus) which just proved I was using it completely wrong (guaranteed I'm using 90% of all warframes wrong anyway since they have to "shine" for me before they get forma or potatoed/reactor) and while I was hiding from the Corpus army doing my sneaky Loki thing he was just tanking them AND the Orb Mother for a time. Definitely a frame I underestimated, just still not one for my playstyle.
Nekros
Easily farmable from Lephantis on Deimos (who is basically a pinata when fighting fairy Titania) the Nekros' main draw is his ability to make corpses cough up more loot. Oh, he can also punch enemy souls from out of them, summon ghosts of killed enemies as allies and scare badddies into running away which doesn't go well with any of his other abilities.
If I needed a word to describe him it would be: boring. For the most part Nekros is a warframe without powers. His punch is weak, the summoned dead expire, and what you will be doing most of the time is double looting corpses. Bleh, not a frame for me. Luckily he's an easy one to reclaim if I ever want to.