737s would be the best option, but DL has already tried the 737-800 on the Shuttle and pulled them off after 9/11 and replaced them with 737-300s. As of right now, I'd say there isn't enough traffic to justify putting a 737-800 on the route; heck, even the MD-80s are a little big (the old 737-300s were configured at 120 seats and US uses 124 seat A319s on their Shuttle flights). However, the E-Jet series doesn't have enough capacity to handle peak Shuttle flights which is a downside (although the E-170 does operate the first DCA-LGA Shuttle of the day due to that plane being quiet enough to do a pre-7 a.m. takeoff out of DCA while the MD-80s aren't.