In principle, H.264(AVC) is the best; VC-1, XviD, WMV2 are the second; DivX is the third; RV10 is the worst so we never upload it. However, encoding parameters are also important, and bitrate plays the most important role. You cannot expect a 500kbps AVC video can have better quality than a 1800kbps XviD one. If you don't think so, you have to buy a larger monitor and try to play those films in full screen mode. Bitrate means how many bits are used to present 1 second video, i.e., 29.97 or 30 frames for NTSC videos. Without enough bitrate, you will lose the detail and even some colors -- you have to understand nothing is free. A 120 minutes film with 3000kbps means the size of its video part is about 3000kbits/sec x (120 x 60)secs = 21600000 kbits = 2700000 bytes = 2636.71875 MB. It has wonderful quality but its filesize is a bit large.
Nowadays, many online shops sell films with VC-1 encoding. Due to some reasons, some people re-encode them to other formats, such as H.264 and XviD. Although we said H.264 is better than VC-1, it doesn't mean the re-encoded version can be better than the source. You have to know the source is always better than re-encoded one.