I've been playing a lot of games made by Valve (L4D, L4D2, HL2, HL2 Ep1, HL2 Ep2, Portal, Portal 2, TF2, etc.) and I can easily say that they make some of the greatest games. The story of each game is very impressive, and the link between HL and Portal keeps it really interesting. It also gives way for lots of easter eggs. I also think that they are really good at making coop games. L4D games are great online, and playing as the special infected is really fun. It's nice to be the predator for once.