For me, my favorite Rainbow Dash-centered episodes are:

1. May the Best Pet Win - cute Tank! Great song! A contest theme (I love those)! Like it was made for me.

2. Testing, Testing 1, 2, 3 - I rank it so high because it explored an interesting side of RD and made her quite self-conscious, but also funny and cute.

3. Sonic Rainboom - it was glorious, what can I say?

Honorable mention: Sleepless in Ponyville. It's not really about RD but I liked her portrayal in it.

For my second favorite, Rarity:

1. Sweet and Elite - that song. And everything else.

2. Sisterhooves Social (if that counts) - it had one of my favorite touching moments of the show, between Rarity and Sweetie Belle.

3. Suited for Success - the song, again, and I loved both fashion shows.

Since I like the rest of Mane 6 equally I won't rank their episodes, not now at least.

Now, your turn :D

P. S. my overall favorite episodes? Probably still Cutie Mark Chronicles and Sweet and Elite. And I can't believe it's been years, man.