This is a thread to specify why you love a game so much. Megaman X5 has to be my favorite of the series, if simply for the opportunity to fight Zero to the (almost) death. That whole game from start to finish was made of win.
Mega Man X was my favorite of the series. It was the first Mega Man visit to the NES. I have Mega Man X2, but unfortunately only rented Mega Man X3. Mega Man X2 has never pulled me into the game quite the way MMX did.
I loved the idea that Dr. Light had left pods with upgrade abilities behind for X. It was refreshing that, unlike prior Mega Man games, you could get abilities that could increase your agility, armor, and weapons without having to beat a boss and drain the weapon energy bar.
Going back a bit, I am still a huge fan of Mega Man II and III for the NES. Mega Man 7 on the SNES has to be one of my most hated games; I beat it once (after countless tries) and never wanted to touch it again!
X1 and X4 for the superb soundtrack. A Rockman game with average to poor music is not a Rockman game so I didn't enjoy X5 and X6 as much as the others despite the games being identical.
Also X1 and X4 got the best and must interesting story and event IMO.
Rockman loss against the first boss later saved by a new Rockman like character. Story with love and betrayal in X4. and reference to the original Rockman game in X1 intro and Dr wily appearance in X4. they aren't really much but as a HUGE Rockman fan these event were really exciting to me.