I'm genuinely surprised at how many people want a Rarity spin-off.
Rarity may be my favorite member of the Mane Six, but I'm really skeptical about the idea of a spin-off based around her. Seems to likely to me that it would devolve into a sort of prime time cable sitcom but for kids. . .maybe.
Either way, Rainbow Dash seems like the most obvious choice for a spin-off series given her personality (she's essentially a shonen series protagonist in the wrong show), the fact that (other then Rarity) she pretty the only member of the Mane Six who has goals and desires that lie outside of Ponyville, and she probably has the biggest cast of characters associated with her and her stories outside of Twilight. Most of the work is already done, you just have to expand upon it. Hell, it doesn't even need to be adventure based like people think, it could just be sports based, or weather team shenanigans.