Who do I take to r5 as my first?

Sergeant_

I'm a Cav player who's focusing on Act 5 100%. I've not started with 6.2 btw. These are my current r4 champs who I can take to r5. I also have a few r1 6* namely Domino, Elsa, Ultron, Man-Thing, Blue Cyclops and Ronan Feel free to give reasons on who I should rankup and why!

Ercarret

I can't say which specific champ is better for Act 5 since it was so long since I played it myself, but I can give some general thoughts about the champs. The only one I wouldn't recommend ranking up is Mysterio. Not because he's a bad champ, but because the rest are just much better.

Havok is great. He can initially seem like a bit of a one-trick pony with his massive SP3, but he's actually really versatile. He does have a massive SP3 when fully ramped up, but he can also deal some insane damage over time by throwing heavy attacks at his opponents. This allows him to place...something, I can't remember the name, but it makes each plasma charge on the opponent deal damage. So the more charges they have on them, the more damage they take per tick. This is a great thing to utilize while building up to the SP3, or if you for some reason can't use an SP3. He also has pretty high energy resistance and places a plasma charge whenever he takes energy damage, so he can cheese a lot of nodes where you take constant energy damage. There was one Act 7 node recently where you got a small shock on you whenever you struck the opponent and Havok just melted them down.

Corvus is amazing for the content you're at right now. He loses a fair amount of value in endgame content since his ability to just completely blitz down opponents is countered by their massive health pools, but that shouldn't be a problem in Act 5. I think. Anyway, he is just an amazing option for so many fights and the more missions you complete, the better he becomes. His awakened ability doesn't do much above sig 1, but he becomes fully immortal with just sig 1. As long as he has glaive charges left, he can't be killed. You can place 1000 bleeds and poisons and incinerates on him, and he won't die. It's amazing. When you run out of charges (or are knocked down), he'll die immediately, but he's so good that you often don't have time to run out of them.

Falcon is probably my favorite buff since Kabam started ramping up their buffing last year. He can shut down pretty much every single defensive ability in the game, which is just amazing. When he's locked on, Nick Fury doesn't gain any charges, Thing doesn't gain any rock stacks, no champion can evade him, he doesn't take damage from nodes like Biohazard where there's a chance that you are inflicted with poison or bleed when hitting the opponent or their block. The list is endless, and it's awesome. I love that dude.

iBom is a new favorite of mine. I just ranked him up a while ago and he's just put in so much work for me. He does benefit a lot from the Willpower mastery, though, since he always has poisons on him and will continuously heal from them. But even without them, he just wrecks. Whenever he blocks an attack, he places a weakness debuff on the opponent which makes their subsequent attacks less powerful. That can be really valuable if you time it right so that a tricky special attack, that you need to block, deals less damage to you. But other than that, he just poisons his opponent into oblivion. Eventually you'll build up enough poisons to acid burn the opponent and then you can just go ham on them. You become fully unblockable and can just pummel them into oblivion. So much fun.

I hope this helps you in some way. I suspect that Corvus' uncomplicated, unrelenting damage (for 20 hits) is an MVP where you're at in the game, alongside Falcon's "shut down everything" mechanic. But I can't really say for sure who would be the best rank-up.