RCA DRC616N 7-Inch LCD Portable DVD Player with 3+ Hour Battery
Best on the Market for the $
I did a lot of research and love this DVD Player.  It has all the features you'd want and the battery snaps onto the bottom rather then being a big plug in thing on the side, the plug in cord is long, long battery life (it's even longer if turn brightness down), child button lock, and all the controls are on the player so you don't have to use the remote (unlike the Panasonic one).  Carrying/display  case is added bonus so you can hang the player from the back of a seat.