Try Megaman X. That would be a good start. Megaman 1 would be awesome too.

Megaman 2 or/and 3

Megaman X

Mega Man X would be my choice.

My suggestion is a firm, "no, pick something else." Are you a monster? Why don't you buy him a Sonic game while you're at it and double the potential for sorrow?

Either Mega Man 2, 3, 7 or X! You can't go wrong with any of those.

Megaman 2, 3, or 10 (via the Wii shop channel). Those are the easiest 3 that are also really enjoyable. 3 the least so of those because there's the Doc Robot stages that kill the pace. 4, 5, and 6 are also really easy but they're a little more bland and the former two of those three also have the filler sections at the end.

Source: I beat all 11 classic Megaman games in the last couple months.

My pick is 4 or 6. Mega Man without the Mega buster and slide is not Mega Man. I'd go with 6.

Definitely not X, though. X is neat, but only after you're learned to appreciate the classics first.

6 really was a cakewalk, but it was also fairly fun to play, so I'm actually gonna push that up with my other three suggestions. 

2 or X are my favorites so suggest start there.