It´s really hard to say.
The original series was just,..well, the original, which introduced us to the whole concept of robot masters and gave us so many loveable character like Rockman, Blues, Forte the robot masters, rush and so on.
Although almost every new game was more of an add-on than a really new game, they never bored me.
Rockman & Forte is my favourite original megaman game.
The X series gave the Rockman universe its speed. Oh, and Zero, a overrated, but still cool guy ^^ I like the X series for being more mature and for its quick and action packed gameplay. X3 would be my favourite.
The Battle Network series was very good as well. It was something totally different and it worked.
I like the battles and the characters in the game are clichee-filled but lovable.
It gives me as an old-school megaman fan a hell of nostalgic vibes ^^ Gotta love the redesigned robot masters.
Too bad, that the series after BN3 went downhill...BN3 was my favourite.
Rockman Zero was just great. It was even more mature and even more speedy than X. I love Zero for the same reason I love the X series. They threw in some nice gameplay changes in this series an I liked them.
Zero 3 is my favourite ( mh..always part 3? ^^; )
ZX looks promising, but I can´t judge it, neither can I judge the new BN series
I´ve never played legends but from what it looks it doesn´t seem to be as good as the other Rockman games.
I voted for Zero because this series need some votes, but I don´t know really ^^;