If you haven't completed act 5 and haven't done cavalier i would say Corvus. Simply because he can slice through most stuff in act 5. And he can do serious damage to ultron even if he dies. He also handles sentinel well in act 6. Other than that, CAIW if high sig. Quake maybe if you can quake well.
I would rank up magneto and quake. To be honest, your current champs could clear all of act 6.
I would rank CAIW, Red Mag, Colossus, OR, and Corvus. Torch will help out with some stubborn fights/bosses.Go in the order you feel you play the best because any of them will be extremely helpful in Act 5 & 6.Personally I run Ghost + Wasp. Then 3 other useful champs for the lane I'm taking or specific needs for the boss
A lot of others have given advice on all the champions you could rank up so I’d like to give advice on specifically getting cav and who you should rank up for that purpose. Rank up your warlock to deal with the crossbones boss and then you should breeze through cav.
Corvus is all you need