Mega Man 2 since it was the first MM game I completed as a young one. I do adore the rest of the Classic series from MM3, MM5, MM6, MM7, MM4, MM8 even the original MM. As for the later titles MM9, MM10, and MM11 they're decent in their own ways. Anticipating MM Dual Override.
Mega Man X2 similarly since it was the first MMX game I completed way back when. But I do also love MMX, MMX3, and of course MMX4. MMX5 and MMX8 are also solid. I understand the sentiment on MMX6 and MMX7 but tolerate them for what they are- although they're near (if not bottom) of the series.
As for spin-offs I recently played, finished:
Mega Man & Bass which was incredible, I will go with that. Runner-ups will be MM Powered Up and MMX Maverick Hunter. Others were good like MM Legends (MML2), MM Power Battles, MM2 Power Fighters. The rest from MM Battle & Chase, MM Soccer, MM That's Paradise, and especially MM Super Adventure left much to be desired.
I've only played half of the handheld games from GB, GBC, WSC, GBA, NGP/NGPC, NDS, PSP. And a few collections on GEN, PS2, GC for example. And haven't played MMX Command Mission, MMX Dive, or Street Fighter X Mega Man yet.
That's where my interest for MM franchise ends. As for neighboring series Zero, MMZX, Battle Network, MM Star Force and other spin-offs like MM Network Transmission I've tried getting into them to no avail.