Panasonic DVD-LA95 9-Inch Portable DVD Player
Very high quality
Other manufacturers like Apex, Initial and Shinco make decent portable DVD players at a lower price, but Panasonic products are simply the best in terms of quality.  From sharp pictures to booming sound (even from the built-in speakers) to build quality, you can't do better than this.  The price is high, but you do get what you pay for.  If you plan to watch DVDs on the road only occasionally, an Apex or Initial would suffice and you do get a lot of bang for the buck.  If you watch DVDs a lot, though, I highly recommend the Panasonic.  I have this and an Initial product and trust me, the quality difference is just huge.