On a grad-student budget, I find that AirTran to BWI is kindest on my wallet. The other perks of AirTran are:
- relatively easy to attain elite status
- free same-day standby for any flight (can't do that on AA!)
- easy trade in "miles" (points, actually on FL) for award tickets
- decent customer service for a budget airline. I've never had to wait more than 5 minutes to talk to a real person via phone.