Mega Man (known in Japan as Rockman) is the protagonist of the Mega Man series by Capcom. He was originally a lab assistant robot, but was later turned into a fighting robot to protect the world from the evil scientist Dr. Wily and his evil Robot Masters.
Mega Man's main abilities are his Mega Buster, a cannon built into his right arm which can shoot powerful plasma blasts and the ability to use multiple special weapons he obtains from the enemies he defeats or in some cases found lying around (such as the Magnet Beam from the first Mega Man game).
In M.U.G.E.N, Mega Man has been made by several creators. Most of them are based on his appearance in the fighting game Marvel vs. Capcom, with JinKazama's being the one more similar to his original incarnation. Others, despite using sprites from said game, have completely new movesets and color palettes.
Though the first and not the best super fighting robot, this one has elements from Marvel vs Capcom combined with an original Level 3 hyper and a Protoman striker.
NES Mega Man is a version of Mega Man created by Hsiehtm, who is based on his appearance in the original games for the Nintendo Entertainment System (known as the Famicom in Japan). This version plays exactly like he does in his 1-6 and 9-10 games, specifically Mega Man 2.
He is able to shoot bullets from his Mega Buster, and use three special weapons from Mega Man 2: Metal Blade, Air Shooter and Time Stopper.
Old School Megaman
There is another version of NES Megaman that can fight, and uses Beat, Rush and Eddie as strikers. He plays in a MvC type style and has decent AI. He doesn't have any specials, but one hyper.
While not a bad or good Mega Man, this is one you should try out if you don't like his Marvel vs. Capcom moveset. He's now more like a shotoclone, of sorts.
Another MVC-Styled Mega Man, this one is generally loyal to the original game, but has new abilities such as calling his robot friends as strikers, and more cooperation with his robot dog Rush, allowing him to transform into a coil to make Mega Man jump higher, or into a jet that allows Mega Man to fly in the air.
But the main feature of this Mega Man is a special transformation, which allows Mega Man to merge with Rush and gain a completely new moveset.