My shortlist (in order): Cable, Colossus, Black Widow and Cyclops.
Any combo of two will do.
Are there arguments to be made for Black Panther (seems good value)? How about Thor, Captain America, Wolverine or Jean Grey? Worth the price bump?
Can't bring myself to even buy the cheapest costume (600G I believe) if it can unlock a playable Black Widow instead. I hear she has variety at least.
I'll chime in...to give you perspective, I'm playing the following characters:
Black Panther - 35
Rocket Raccoon - 21
Colossus - 20
Jean Grey - 18
Daredevil - 10
Punisher - 10
Various others (to include Storm, Hawkeye, and Deadpool to 8. I have Thor and Cyclops from a pack and haven't touched them.
Feedback for the ones you asked about:
Cable - Ridiculously solid range class from my observation. Some say he's overpowered. His main stats are currently bugged and don't level up like the others, so he'll probably be even better one that is fixed.
Colossus - If you're already playing Thing, I would suggest steering clear of Colossus. He's arguably the best tank in the game, but there will be many similarities in play style.
Cyclops - Looks amazing. Some are arguing he doesn't hold up as well at top levels, but the ones I've teamed with have obliterated everything. His ultimate is currently broken and one shots most bosses.
Black Panther - My main and an absolute blast to play. He's quick, insanely survivable (I went 50-1 in PVP last night, oftentimes against three players at once...and I SUCK at PVP most games). His main strike is a ridiculous upper cut plus bleed effect that is borderline OP. I've added traps into my latest build and he's even better now. With 40% dodge rating and climbing, you can really mix it up. He had the highest skill cap of any character I've made.
Black Widow - Really fun play style mixing melee and range, and quite good survivability of you pump up her dodge passive. She's the only character that gave me any issues on Panther in PVP...still beat her 10-1 but I'm convinced Panther is OP right now in that realm.
Jean Grey - Starting in the late teens, she's become an AOE monster. I kill regular enemies faster with her than any other class, by far. Phoenix form is visually stunning, and as long as you keep a steady stream of bad guys coming it's basically endlessly sustainable. Where she lacks is in killing power against bosses and she's not super survivable.
I'll defer on commenting on Wolverine, Captain America, or Thor since they don't interest me and I haven't partnered with them much.
*One final note: melee characters suffer a harder time currently in many boss fights than ranged ones do. That may or may not change in the future, but it's definitely worth considering.