I'm going to start the new year by sharing five of my favorite episodes from each season of FIM (five episodes from Season 1, five from Season 2, and so forth). And I'm obviously going to start with Season 1.

5.A Bird in the Hoof — I love how determined Fluttershy was to curing Philomena, even when it seemed near impossible. The Benny Hills-style chase scene was also quite hilarious to watch. I REALLY wish we got to see more of Philomena beyond this episode.
4.Applebuck Season — Applejack's quite a hardworking pony, and this episode shows that everyone in Ponyville admires her for that. However, this episode also shows that Applejack can be a bit stubborn, and her reluctance to accept help results in sleep-derived behavior that affects EVERYONE.
3.Feeling Pinkie Keen — This episode establishes the cartoony, comical character Pinkie Pie is well known to be. It also establishes one rule about this show's fandom: Don't. question. Pinkie Pie. Twilight learns this the hard way, being subject to a lot of the misfortunes that occur throughout this episode.
2.A Dog in a Pony Show — Previous episodes would depict Rarity as a snooty, self-centered pony who only cared about her appearance and/or showing off (Sonic Rainboom and Look Before You Sleep are prime examples of this). But it's shown in this episode that her dignity comes first. Not only does Rarity show disgust at the Diamond Dogs for treating her like their slave, but she also lets loose some high-pitched whining that eventually forces her kidnappers to let her go.
1.Stare Master — This episode and Dragonshy were the first glimpses we got of Fluttershy having an assertive side. We're also introduced to "The Stare," special kind of glare only Fluttershy has which she would occasionally use in future episodes. Props to Fluttershy for defending the Cutie Mark Crusaders against a cockatrice while she's being turned to stone!

Disclaimer: These "top 5" images are my opinion, so you're free to agree or disagree with them.