Awesome OP.
For me, it's classic and Zero basically tied, with Legends as the runner up.
Classic doesn't have a single bad game in it, and also has the most entries if you don't count the slightly different versions of EXE/BN games. 7 and 8 were only 'good', but 9 is the best in the series and 10 is very fun as well.
Zero only has 4 titles, but all of them are fantastic.
Legends is amazing, but unfortunately Legends 2 is one of ~5 Mega Man games I haven't played, so I can't really judge it as a series. Still need to pick it up some day, but I know as soon as I shell out the cash for it it's going to go up on PSN. Still regret passing it up back in the day because it was pretty expensive at ~$40 whenever it showed up at EBGames. I can't believe we live in a world where Capcom cancels Legends 3 and releases Breath of Fire VI. It hurts.
Mega Man X is by far the most overrated series. X is very good, X2 and X4 are OK, X3 is kinda bad, and everything else is basically terrible. Everything this series does, Zero does better.
Battle Network is also fantastic, but, though the games systems do change between titles, I get pretty burnt out with all the stupid dialogue and bad 'dungeon' design. The battle system is one of the best in the history of RPGs to me though.
ZX is overlooked I feel, and ZX Advent only passable. That series had so much promise... I really wish ZX3 hadn't been cancelled.