The strong guy of the FF. Rock-skin, near-Hulk strength... I think that covers it.
Megaman X
Megaman from the FUTURE! Wields X-buster and ability to copy defeated enemy's powers.
(Potential for abuse and twinking: large)
Zero
Robot from the same universe as Megaman X. Wields sword sabre (ala Gundam) and bishounen looks.
KOS-MOS
Xenosaga android, IIRC. Haven't played the game yet... nor do I have a PS2. ;_;
The Atom
DC universe character. Not sure if this is the Silver or Golden age version.
Golden age version: Short powerhouse. Acrobatic abilities. Think of someone who's shorter than you, but can kick your ass 'til next Wednesday.
Silver age version: Ability to shrink. Can shrink small enough to ride electronic signals, distort brain patterns (if he gets inside your head), etc. Underrated, but powerful. Ray Palmer is a scientist, too, so expect that he'll be using his powers to the maximum by playing smart.
The Doctor (pending)
Who?
(Bad pun, I know. :P)
The Flash
Three versions available: Golden Age, Silver age, Modern age
Golden Age: Jay Garrick. Can move as fast as speed of sound, close to speed of light, IIRC. Able to do "speed tricks" (multiple punching, create wind tornados with hands, etc.)
Silver Age: Barry Allen. Can move as fast as light. Can do speed tricks (vibrating through walls, create wind tornadoes with hands, etc.)
Modern Age: Wally West. Can move as fast as light (formerly only speed of sound). Able to do "speed tricks" like Barry. Can share his speed with objects or people in contact with his speed lines.
Silver Surfer
Former herald of Galactus. Wields the Power Cosmic. Acts a kind of a cosmic battery, able to absorb energy from cosmic sources (stars, nebulas). Uses energy to wield powerful energy blasts. Super strength and near invulnerability. Has surfboard to cruise space.
Firestorm
Able to change the molecular composition of atoms. This means he can change oxygen to hydrogen, carbon to cement, water to diamond. Sounds like a twink? Fortunately, he has to know the chemical composition of the item he's changing and the item he's changing it to. Can you imagine fighting and doing chemical computations in your head at the same time? :/
Also, be aware of this fact (if you're playing them as canon characters): old Firestorm (70's-80's) was really two men who changed to Firestorm, a jock Roy Raymond and a Professor Stein. You can see that Stein is the brains behind the powers. Recent Firestorm is only Roy Raymond, as Professor Stein became a fire elemental and left earth, so he's less adept at getting chemical compositions right.
Captain America
Right. As if you didn't know. Higher than average strength, athleticism, and stamina caused by Super Soldier Serum. Wields aerodynamic shield that always boomerangs back to him.
Superman
Christopher Reeve forgetting to wear his briefs under his pants. Super strength, flight, invulnerability, heat vision, super speed, super breath...
Wonder Woman
Amazon princess. Flight, super strength, invulnerability (not just the bracelets anymore), lasso of truth (which has limited hypnotic properties).
Orion
Son of Darkseid. From what I know, he is super strong, near-Superman level invulnerable, and a super hothead. He also goes to battle in a glider that is able to shoot laser beams.
Batman
World's greatest detective, expert martial artist, escape master, master strategist, utility belt. No super powers, though. Potential for uber-twinking if based on Morrison characterization. Under Morrison's JLA, Batman is not only omniscient, he's also apparently some sort of super ninja. :/
Has been depicted as able to beat Superman (or nearly everyone) if given enough time to plan and set up countermeasures.
Captain Marvel (DC)
The big red cheese. Possesses the wisdom of Solomon, the strength of Hercules, the stamina of Atlas, the power of Zeus, the courage of Achilles, and the speed of Mercury (all of which spell SHAZAM!). Basically, Superman without the heat vision. When he utters SHAZAM!, he returns to his alter-ego, a kid named Billy Batson. Billy must shout Shazam to become Captain Marvel.
Hmmm... a JLA team. Interesting.
Adam Warlock w/ Soul Gem
The Soul gem has the ability to steal souls. Adam also uses it to launch karmic blasts, a beam of energy from the Soul gem. As for Adam, I think he's of metahuman strength, invulnerability... but, that's it. He was created as the perfect human, IIRC.
Cloak
A being of pure darkness. In shadow form, he can merge with shadows, envelop enemies in darkness... can't recall other powers, though.
Absorbing Man
Able to take on properties of items he comes into contact with. For example, if he gets hold of a brick wall, he can change his skin to the texture of bricks. If he gets hold of a metal bar, he can change his skin to be as hard as that metal.
Black Adam
Think an Evil Captain Marvel. Black Adam has to say Shazam backwards, though, I think. (Been a while since I read him.)
Terrax
A herald of Galactus like Silver Surfer. Also wields the Power Cosmic so has most of Silver Surfer's abilities. Has big axe instead of surfboard.