Which 5-stars to level up

Lemminkäinen

I think I'm ready for the giant leap. I have all but the most recent ten or so four-stars champed and the biggest ones are 320+.

I have the following five-stars at 13 covers:
Thanos, Black Panther, OML, Doctor Octopus, Strange, Ghost Rider, Iron Man, and Thor.
All are level 255 except Thanos is 330 and OML 323.

The following are at 12 covers:
Daredevil, Hawkeye,Phoenix, Silver Surfer

I have 3.5M of ISO.

Should I make the leap? And if so, which ones should I champ? All input appreciated!

Lemminkäinen

OJSP said:

I'd start with Thor. How's your Okoye/Shuri?

Shuri champed but Okoye is really undercovered (something like seven covers, I think).

Lemminkäinen

Kitty is at seven. I also do have Bishop champed, if that matter any.

CT1888

How much pvp do you play? 

If you play predominantly pve, champ Thanos and never look back.