Well, I like all of them, but if I had to play favorites, it would go in this order...somewhat.

1st Place- Legends
1st Place(tied)- Battle Network, but only 3, 4, and 5.
2nd Place- X series (MM Zero is somewhat a story of the X series in itself, so they are the same to me)
3rd Place- the original RockMan series.

Although I have never played Legends 1, 2, or Misadventures of Tron Bonne, I have played Megaman 64 on emulator... Tron is just too hilarious to be in second or third place.

The series that really made me like the Megaman series was the battle network series, just for its ability to be fun, yet however to remain so simple.

X1 was really the first MegaMan game I ever really played through... at that time, the only one I had played before was RockMan 4, and that was just too hard for me. (God, give me a break... I wasn't even 6 years old yet!)