Op For is my favourite of the series, wish they'd make a sequel for it.
I did enjoy HL2 and eps 1 & 2, but only on the first play-through. I've tried to replay them but found it nowhere near as enjoyable, because I already know what happens. For a singleplayer FPS game I found the story surprisingly engaging, but that really is the only thing that kept me interested.
Still eagerly awaiting ep 3, though it seems it has the same release date as Duke Nukem Forever and Starcraft: Ghost