My picks are below (in no particular order except for the first two):
Mega Man V (Game Boy) - this one check's all the right boxes for me, with good music, interesting bosses, solid level design, and the only Classic Mega Man game where Wily is NOT the final boss (no, not a joke, very serious!)
Mega Man X (SNES) - great music, great level designs, solid controls, even challenge, and secrets abound. A very solid game.
Mega Man X6 (PSX) - easily one of the WORST Mega Man games just for level design alone, with a horrid engrish translation and badly explained game mechanics, this should not be an enjoyable game... except that I do find it cathartic to play thru with the Ultimate Armor code as X, the music is great, and the engrish translation does evoke some laughs here and there
Mega Man Zero 3 (GBA) - my fave of the Zero games, the game is still hard as heck (as all the Zero games are), though more enjoyable than Zero 2 (which at times was borderline unfair). Good Music, neat bosses, and an interesting final boss. Cannon Ball!!!
Mega Man II (Game Boy) - this is one of the worst of the portable Mega Man games, though on average it is superior to most platformer games (the controls work, few cheap deaths, etc.). The worst aspect is the music (a programming oversight caused the music to play incorrectly), but the game is a fun play to kill time and relax.
Mega Man X4 (PSX) - the last "good" X game (until Maverick Hunter X on PSP), good music, decent stage design and Great/Awful voice acting. Tremble with Terror, You Are Finished!!!
Mega Man III (NES) - my favorite of the NES games, MMIII had good stage design, great music, and an interesting gimmick of revising levels and adding in bosses from the previous game. Oh, and a BUSTED, BROKEN MESS of a Boss weapon in Top Spin, that either deals no damage, a bit of damage, or INSANE damage, usuing either some, all or NONE of the weapon energy...
Mega Man Legends 2 (PSX) - the best of the Legends series, and a good 3D Action game in its own right. With cheesy anime-style writing/action, fun levels (minus the Water Ruins, arguably worse than Ocarina of Time's Water Dungeon), and good music, this is just fun.
Mega Man 10 (Wii/XB360/PS3) - a fun "homage" to 8-bit Mega Man games, superior to MM9 in that the stages are FAR less Trap/gimmick heavy. Oh, and Sheep Man.
Mega Man Battle Network 3 (GBA) - of the Battle Network games, this is the most balanced of the games before the series went the full Pokemon route with significant differences in the version releases (example, BN4 had different routes/story/bosses per version). Oh, and get the Blue version for the Punk Boss (based on Punk from Mega Man III Game Boy).
