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.