Of those listed, I chose Garrys Mod. But if you're looking for best that aren't on the list, check out some nostalgia with
Golden Eye: Source, a remake of the multiplayer Golden Eye for the N64. I'm personally waiting for
Black Mesa: Source to be released later this year. It's been in planning since 2004-2005, and they're finally getting ready to release it to the public!
If you're also including any old mods, you're going to have to include Counter Strike, and some of the early Team Fortress mods (though the only one I can fit in this category would be
Fortress Forever, a remake of the now Team Fortress Classic).
There are thousands of mods, some of which are basic, and many of which are entirely new games. I love the Source engine. One of the best investments in gaming someone can get.