Here’s a basic summary of character types. Note that every character has some fluidity; for example even Moon Knight (a tanky melee oriented character) can easily be built hybrid style that craps out ranged damage effectively enough to clear trash enemies even at the Cosmic level. Every character has some area damage (they have to, you’ll be taking on a dozen enemies at a time pretty frequently)
Note: power rankings and such on this assume that you want to be able to competitively play at the Cosmic level. If you don’t care about this, I’ve got great news: you can just totally ignore everything I wrote about power levels and just pick whoever sounds interesting.
TANKS - They get in there, they can take some hits, they dish it out. Toughest dudes in the game, right here, although they are by no means invulnerable as a couple of high damage boss attacks will still make them worry. They can probably take it, though.
Colossus, a melee guy. Tough, hits decently hard.
Moon Knight, hybrid, with an emphasis on melee, gets more dangerous as he loses life. Also has the highest single target damage in the game.
The Thing, the prototypical tank that unfortunately doesn’t do as much damage as he should (BUT is slated for a rework). He’s really, really tough though.
Hulk, he’s tough. Not very strong but getting a rework.
HIT AND RUN - these characters get in, do damage, and try to avoid retaliation through stealthing or some other means
Black Panther, hybrid or melee character with a splashing of area stuff, uses stealth, has good single target damage
Black Widow, supposedly hybrid, uses stealth, haven’t played her at all. Mixed reports from my friends on her.
Nightcrawler, awesome weird melee hybrid who teleports around, leaves damaging zones, and kills stuff. Uses stealth plus teleports
Invisible Woman, turns invisible and uses her decent area damage to rock stuff. Is also great in a party as she adds buffs. Needs a buff.
AREA NIGHTMARES - the whole screen is covered in damaging zones, enemy life bars melt away, welcome to Thunderdome. These are ranged characters which means they are a bit squishier, in general, then melee/hybrid characters and a lot squishier than tanks.
Dr. Strange, hard for new players to use. Requires a lot of execution. Put on a laser light show and yell about the Butt Blasting Breakfast Burritos of Bruce Banner. Really powerful though, if you can handle 2 full bars of skills.
Storm, just got a rework and is in a great place.
Human Torch, when built correctly this dude is top tier.
Scarlet Witch, she’s pretty good.
JUST SORT OF RANGED DUDES - they have ranged attacks, tend to be squishier than the melee guys.
Hawkeye, can get ludicrously high Critical rating. Shoots arrows.
Punisher, has lots of weapons, not in a good spot, will be getting a rework.
Cable, he’s ok and a great terminal clearing machine, but getting a rework.
Rocket Racoon, has two main builds: turrets and guns. Also summons Groot (who is ok, don’t hate on Groot man), got his rework, still not great. Stick to using turrets as his guns are anemic.
Loki, has illusions, a teleport, and can put a lot of different effects on enemies. Not very tough though.
Silver Surfer has some good ranged, some good movement powers, and you know what? His area power ain’t half bad either. He also has a channeled single-target beam that can give Moon Knight a run for his money if you build around it. Interesting character.
Iron Man is a solid all-arounder that mostly plays with ranged/area effect stuff, and he’s pretty tough to boot. He also has a way to turn his maximum Spirit level into more damage, which gives him some very interesting gear options.
Star-Lord has got ranged damage, AOE options, and can frequently pop off and do tremendous melee damage to a single target. He’s a beast. Just stay out of the Elemental tree; at the time of this writing (Sep 8 2014) it’s garbage but the developers have promised to look at it.
JUST SORT OF MELEE GUYS - they get in there, and tend to be tougher than the ranged guys. They are distinguished from tanks by generally doing more damage (or having more area effects) but not being quite as tough.
Daredevil, melee dude. Solid character, no real flash, but gets the job done.
Luke Cage, melee with a smattering of summonings including the ability to call in 4 Heroes for Hire at once for a short period of time. Has one great area power too. Don’t be fooled, though - you should play and gear him as a melee powerhouse.
Captain America, melee + shield throw, pretty tanky actually. This was a tough one, I didn’t know where to put him.
Wolverine, he’s the best there is at what he does (which is stacking bleed effects on people) and is pretty tough, too, if you give him a moment to regenerate.
ONE-TRICK BUILDS - these characters generally have one strong way to be played and woe betide those who don’t enjoy it. Again I stress: if you don’t care about Cosmics you can do whatever you like, just read the last sentence.
Special note: as of Sept 8 2014, every single one of these characters is getting reviewed/updated in order to give them viability. So that’s good!
Ghost Rider - Whirling Chains. Stands still and whips a chain around. He’s supposed to be a movement/Area Nightmare character.
Cyclops - One Eye. One Button. And that button is Ricochet Shot. Supposed to be a ranged character.
Jean Grey - Phoenix Form, stuff just dies while you sort of float around. Supposed to be a hybrid character who switches form.
HYBRIDS can be built ranged or melee, and tend to (but not always) work best as a combination of both
Spiderman plays well as a movement based wacky hybrid. Has a crazy powerful bosskilling punch that can brutal for 500k.
Emma Frost us a minion-using telepathic ranged character (or diamond-skinned melee brawler, or both! Although apparently that’s a pretty tough build)
Deadpool is hybrid or ranged. He’s also annoying.
Taskmaster, can be built deceptively tough in both ranged + melee department or hybrid. Is also only 200 splinters. My advice: buy Taskmaster as your first splinter character.
Ms Marvel, she’s cool.
Thor, he craps damage if geared correctly but doesn’t play like you think Thor should. He’s hard to level, too.
Psylocke, I haven’t really messed with her.
Gambit, he’s got ranged and melee! And summons Rogue for his Ultimate.
Mr. Fantastic has got melee attacks with huge range, some beam weapons, some area debuffs, etc. but he’s best when he is bouncing around and stomping on people’s heads like Mario. Probably need to make a “movement” category of hero and put him in there with Spider-Man, Silver Surfer, and Hulk.
BEST OF THE REST Odd characters that don’t fit into the other areas
Squirrel Girl can be built as melee, ranged, or as pet focused (but needs the right gear for that last one, so be careful)
Rogue is a character optimization lover’s dream, with over 100 potential powers that let her do anything from be a ranged blaster to melee powerhouse to pet class (?!??!). Probably the wackiest, least standard character in any ARPG, anywhere.
I hope this helps you make your decision.
Edit: Updated Sep 8 2014