Gabe Newell was on-hand for his Reddit AMA a few hours ago, where he was asked a slew of questions - with one of them asking what was his personal favorite Valve game/series.
Newell's response: "I think Portal 2 is our best single-player game. I play Dota 2 the most of our multiplayer games". Surely you're thinking 'but what about Half-Life', and I would agree. Newell continued: "The issue with Half-Life for me is that I was involved in a much higher percentage of the decisions about the games, so it's hard for me to look at them as anything other than a series of things I regret".
Our Lord and Savior GabeN continued: "There's no information in my response about what we'll do in the future. It's simply easier for me to be a fan of things that in which I was less directive. If you are involved in a game, everything ends up being a set of trade-offs. Anything in a game is a sacrifice of things not in the game. I just feel those more personally about Half-Life for a bunch of reasons".
Wrapping up that question, Newell concluded: "And Xen".