There are waaaaay more spinoffs than Megaman and Megaman X.
In no particular order, there have been games to;
MZero, MZX, Battle Network, Star force, Legends, there was even a soccer game once. If this poll is to go somewhere, I'd advise putting up more options.
Annnnyway, not to say that any one of them are better than the others, nor to say that I have an exact favorite, but I grew up with the early Megaman X franchise in-hand. Though the other games are awesome, I'm keeping X1 - 3 right at the top of the Megafoodchain, with 1 as the king. It plays awesome, it's got a rad backstory, and it's still got one of the even harder end-bosses in Capcom's history.