I have been doing some research on the same subject recently to maybe replace my 3rd gen ipod by something that also do video.
The best DAP depnends a lot on what your main objective is (size, price, SQ, Video quality, features etc...)
I find that the flash based DAPs (nanos etc...) usually have a screen way too small for video to be usable for movies or even TV shows, so it starts with the HD players if you want decent video.
If you want something very pocketable, the ipod with video and zen vision M have good SQ but small screens. The new archos 406 has a bigger screen but apparently SQ isn't as good (it seems that SQ problem is fixed by a firmware upgrade, so it's difficult to find a consistent group of reviews when the quality varies with firmware).
If you want to go for something bigger (and more expensive) the Zen vision (NOT M) has the best screen resolution (640*480) but is a bit pricey at €400 for 30 Gb. In widescreen format I'v heard good thing about the cowon A2, but they are difficult to find in Europe. The new Zen vision W and the new archos 606 are also options but I don't know enough about these yet to make myself a clear idea, as these are very new. The archos looks very interesting but I am a bit worried that they seems to require a few firmware updates to get the SQ right.
If you want a small DAP with a big screens the archos 406 should be good if you can get confirmation that the sound problems have been solved.
Personally I'll wait and see what the next Ipod and the zune look like before I make a buying decision.