Classic: MegaMan 7, 8, MegaMan and Bass, MegaMan 10
X: X1, X3, X4, X8
Zero: Zero 2 and Zero 4
ZX: ZX1 for the story, ZX2 for the gameplay improvements
Legends: Legends 1 for the charm and familiarity with the setting and NPCs, Legends 2 for the overall improvements
Battle Network: 1-3 are my favorites for gameplay and plot.
Starforce: Never played them yet, no opinion.