Unfortunately, while VLC and Media Player Classic HT can play DVDs, they do have some glitches to watch out for:
VLC : It does not detect interlacing, so you must manually enable deinterlacing each time you play a DVD. While VLC can be configured to deinterlace by default, this results in it trying to deinterlace even when playing progressive video (e.g. AVI files.) I also have a few DVDs which VLC cannot play, giving errors about the DVD's VIDEO_TS.IFO file.
Media Player Classic HT : As far as I can recall, deinterlacing is off by default on this also, but it can be set to automatically deinterlace. However, it also seems to have trouble playing certain DVDs, but in this case with it freezing, especially with home-made movies. For example, on one DVD I have, the video freezes each time the scene changes. However, on the DVDs it plays fine, it is very well featured.
Zoom player was also another favourite, but it's no longer free (although I don't think the free version supported DVD). Interestingly, while I have the shareware professional version on one of my PCs, it never expired, so it's worth a try if the others give an issue.
I only heard of GOM for the first time above, so can't comment on this one yet, but will give it a try for curiosity.
The main DVD player I use is WinDVD, but it is not free.