>>15223179
You can skip 5, 8 and Bass. They're not bad games, but 5 is unremarkable, while 8 and Bass have some really annoying design choices. If you want an often overlooked classic Mega Man game, try Mega Man V for the gameboy.
With the X games you can play 1-5 without many problems. If you feel like skipping one, I would say X3. X6 is a rushed mess and should be skipped unless you have an insane amount of patience. It does have some really great boss weapons and armors, though. Only play X7 if you feel like torturing yourself. X8 was pretty damn good, though. Xtreme 2 is also worth playing if you want more after playing the main entries.
The Zero series is amazing from start to finish. There's a lot of frustrating elements in the first 2 games, but the core mechanics make up for it. MMZ3 is my personal favorite MM game of all time. MMZ4 isn't as good as Z2 or Z3, but it's still one of the better games in the entire franchise. The ZX games are mostly MMZ5-6 with different transformations instead of elemental chips and special moves. So it goes without saying that they're good too.