Depends on what you use the video-hosting sites for.
Veoh has by far the best quality videos and presentation; you can find almost every anime and TV series. It has also less spam, thing that YouTube is well-known for. VeohTV is an excellent feature since you can pick between just watching it or download it; it is also good to watch movies. The only bad thing is the bad searching system.
YouTube is used to watch mostly funny and famous videos; plus it is more well-knowed than the other 2. It also has way more videos; and, as pointed before, it is used mostly to video blog, thing that is getting more popular nowdays thanks to it. The "bad" thing is that it is deleting anything that has to do with copyrighted things; also the poor quality
Sorry but I don't know what dailymotion is ._.
If I had to pick one of them it would be Veoh by far.