Season 5A has been consistently good.
Season 4 (for the most part) was good, but it still had its REALLY bad episodes and mediocre ones.
Season 3 I think was actually overall pretty good. It has my least favorite episode of the show (One Bad Apple), but it also has some good episodes (Crystal Empire, Too Many Pinkie Pies, Magic Duel, Magical Mystery Cure)
Season 2 had both good and bad episodes with very little mediocre. Thankfully, there's more good than bad, but there's still some bad.
Season 1 is easily the worst, being an "okay" season. It didn't get good until "Suited for Success" and the only episodes I'd consider good to great before then are "Dragonshy" and "Call of the Cutie".
Problem is, Season 5 hasn't ended. This would be a lot easier to answer if it was individual episodes instead of entire seasons.
EDIT:
I think for some clarification, I'll do this:
The # is the amount of episodes that fit into their categories.
Season 1:
Good-Amazing: 13
Mediocre-Decent: 9
Bad-Terrible: 4
Season 2:
Good-Amazing: 14
Mediocre-Decent: 7
Bad-Terrible: 4
Season 3:
Good-Amazing: 8
Mediocre-Decent: 2
Bad-Terrible: 3
Season 4:
Good-Amazing: 17
Mediocre-Decent: 4
Terrible-Bad: 3
Season 5:
Good-Amazing: 10
Mediocre-Decent: 3
Terrible-Bad: 0