I too only have about 20 DVDs so I'll just rate my top seven. My rating will be on the quality of the DVD in terms of quality and quantity of extras, technical specs, how cool the different screens/menus look, and overall ease of navigation. I'm not rating them by the actual quality of the movie.
1) Episode One - Simply the best DVD set I've ever seen. Beautifully done with great features and kick-A looking menus. I hope it's only topped by EP2 and LOTR.
2) Matrix - Great options for all the behind-the-scenes viewing.
3) Planet of the Apes (2001) - A lot of stuff just jammed into the set. The multi-view scene selection is a novel idea. Too bad the movie wasn't that great.
4) Mummy (Ultimate set) - Watching this movie on my set is so awesome. It sounds and looks so good. Plus it also has tons of extra features and 3-movie commentary tracks.
5) Nightmare Before Christmas - Not as many extras as these others, but we get to see a lot of TB's earlier work.
6) X-Men - What I really like about this disc is that you can insert the deleted scenes back into the movie for full continuity. Plenty of production extras plus Hugh Jackman's screen test.
7) Close Encounters - They included tons of deleted scenes that help to sort out some of the earlier movie scenes, i.e., why Roy gets fired, how Bob Balaban gets hooked up with Lacombe. What this movie glaringly lacks is director commentary by Spielberg. How could Columbia let this happen?
I've also seen the Godfather boxed set (my dad got it for XMas last year) and it also is a great compilation. I would include it in my list if I owned it.