With 80-something episodes - the vast majority of which are good to great - it’s tough to pick a fave. It’s even tough to pick a least-fave, because on the rare occasions this show fails, it fails spectacularly.
Favourite:
“Party Of One”, because it’s the episode I’m most likely to go back and rewatch. And I love Pinkamena.
(honourable mentions: Sweet & Elite, Hurricane Fluttershy, Lesson Zero, Sisterhooves Social… a few S4 episodes will be added once time has passed)
Least Favourite: *(please note that this is not the same thing as my “worst” episode)*
“It’s About Time” - I have an irrational hatred for this episode. Mostly because I can’t stand stable time loop stories and its ending is bullshit. Sure, on a technical level it’s not bad, but IAT has the distinction of being one of only a tiny handful of episodes that I’ve never gone back to rewatch. (MotFE is another)
(dishonourable mentions: Rainbow Falls, Mysterious Mare-Do-Well, Spike at Your Service, Games Ponies Play, Putting Your Hoof Down, Mmmystery…)