Classic series I'm plenty fond of the 4th entry for adding what we basically use in other megaman games, the buster charge staple. That and toadman is silly.
X series I love 4 because it feels like an actual cap to its respective series.
Legends I adore both these games, they are my absolute favorite entries in the entire franchise. I grew up playing these with family and friends, the experiences these two games have given me will stick with me for all my life.
Zero's first entry because of the leveling mechanic. I thought it was silly at first but it grew on me, and I missed it in the other entries.
Battle network, 2 all the way. Taking the beta game that was 1 and turning it into a real delight with style changes, and the addition of subchips. To let you curve the battle system.
Star force... what.. no.. go away.