Huge fan of Khao San Rd. Best Pad Thai in the city I'd say. The only down side is they have limited seating so it makes getting a table pretty tough. The wait staff is amazingly friendly, and the food is always great.
The Khao Soi was SOO good, we ordered another one because we were having trouble sharing it. The runner up would be the bang-kok pad thai, perfectly cooked noodles, the flavor is really savory and a bit sweet, perfectly balanced. If you're a fan of pad thai, this is the place for it hands down.