After a few flawed episodes, "May the Best Pet Win!" is quite refreshing. It's a delightfully entertaining episode filled with just about everything you could ask for in a pony episode. Thrills, laughs, character development, a great song, it's all wrapped up in a tidy package that makes for a great episode.
Rainbow Dash's characterization is in top shape here. She's charismatic, entertaining, and sometimes adorable. She does have her faults on display, particularly her arrogance, but these serve the story well, allowing for character development. Rainbow discovers that there are things more important than aesthetic awesomeness, and shows her values in picking her pet, which is a great character moment.
However, that's not the only great thing about the episode. It's filled with great humour and plenty of entertaining moments. Rainbow's "boot camp for pets" is great, and in general her attitude leads to plenty of funny moments. Another particularly great one is when she cues to "pause for dramatic effect", almost leaning on the fourth wall, but it still works in-universe. Little moments like these make the episode constantly delightful.
"May the Best Pet Win!" is thrillingly energetic, boasting excellent forward momentum. The episode is mostly centred around a race, which manifests itself with a fast-paced action scene. It's exciting on its own, and it's made even better by Rainbow's confidence. Meanwhile, the aforementioned boot camp scene contains many entertaining and impressive actions by said pegasus.
And then there's the song, titled the same as the episode. It's also superb, being one of the catchiest tunes in the entire show. It's a joyful little tune with soaring melodies and lyrics that advance the story. It's a fan-favourite, and for good reason: Between the great singing, great composition, and great lyrics, it's just an excellent song.
"May the Best Pet Win!" is the best episode since Lesson Zero, with almost too many positive qualities to count. Nearly everything it does it does well, and it does a great many fantastic things. This is how a Rainbow Dash episode ought to be done... and that's especially notable given what comes right after this episode. This episode, at least, is excellent.