"Hearts as strong as Horses" is still the cutest bit I think I've ever seen. The CMC's are just cuteness on parade.
As far as the Mane 6 - Rarity and Pinkie Pie always stand out to me. Both have very subtle moments that are easy to miss (pay attention!). Applejack also has some really cute moments that catch you by surprise.
I'm also a Cloudchaser and Flitter fan