I've only seen a few episodes on youtube (it really is a better show than it sounds with my favorite being AppleJack
), so what are some good episodes to look up and watch?
Ooh, Applejack fan. She hasn't been getting much focus lately, unfortunately. Only really having major roles in the ensemble episodes. Of course, maybe that's because she is already such a good friend that she doesn't have many lessons left to learn.

For an Applejack fix, you would want “Applebuck Season"‚ “Look Before You Sleep"‚ “Fall Weather Friends", and “Over a Barrel" from season 1 (episodes 4, 8, 13, and 21, respectively). Season two has “Sisterhooves Social", (episode 5), which is primarily a Rarity and Sweetie Belle episode, but features Applejack and Apple Bloom rather prominently.
The best ensemble episodes are the two season premiers (each is a two parter), “The Best Night Ever†(season 1, episode 26), and “Hearth's Warming Eve" (season 2, episode 11). “Winter Wrap-Up" (season 1, episode 11) is also pretty popular.
My personal favorite is “May the Best Pet Win!" (season 2, episode 7). It's a Rainbow Dash episode, but that song just gets me every time!
But, yeah, I otherwise gotta agree with Zachski. Watch 'em all, and find your own favorites!