Half-Life 2, both Episodes. Get the Orange Box if you haven't, it has HL2, both Episodes, and Portal, and it's on sale right now for next to nothing. I recommend you install the base game first, then the Episodes, and then finally play through them. Not only does that make sure the assets are on your system, but they're also really good games that every GMod player should experience. You'll also understand why everyone's wanted Half-Life 3 for so long.