The Black Sheep always tends to be one of my favorites, I've noticed.
Spirit Tracks
Gen 5
Partners in Time
Super Paper Mario (Although nowadays, the black sheep of the series is Sticker Star)
The NSMB series
and a few others
Still though, I always liked how B2W2 expanded on the Unova region's already great story, adding new areas and truly showing how the region and the BW characters had progressed. Fitting, seeing as how the name of BW's credits theme was, "To each future". (Also, awesome credits theme)
Honestly, when replaying B2W2 I notice that it has a lot of things that Gen 6 seems to lack, like Dark Grass and the "Victory is Near!" Music.
God I love that music.
Also, PWT, and remade versions of every single Champion and Gym Leader theme, as well as the moving sprite intros for each character.
Cynthia's theme was remade twice. Once in BW, and again in B2W2.