Ill probably go with Crobat. I looked at swoobats stats and it does look pretty weak.
Sigilyph looks interesting, and I already have a couple from the dream radar, but ill pass on it.
And I was thinking about drifblim, but I already have a ghost pokemon (gengar).
Swoobat's main selling point would be its ability Simple and a move like Calm Mind. Simple doubles the stat changes, so you'll really be getting +2 to both SpAtk and SpDef with every Calm Mind. It makes Swoobat pretty powerful, but it is really weak before it can setup.
Sigilyph and Crobat are both powerful. You can't go wrong with either.
I can never use zubat and ratata because I feel like I'm encouraging them jumping me every five seconds in caves in generation 1.
Best feature for BW2 is the auto-prompt for using another Repel when one runs out.