Not sure what it was that got me into it. I never played Blue Shift or Opposing forces (although now after writing this i might just duck back in time and play through them as well)
but i did play these games from the first HalfLife, then HL2, Ep1 then Ep2. So i bassically started at the very beginning and played through till the end. Playing that way will get you into the story line. Also it was quite a few years ago now that i played the first HL game and back then it was pretty well up there. Fire it up these days after playing games with DX10 and the likes will probably make you wonder what all the fuss is about. As for a fav out of the series, probably the first, but each one gave me hours of fun.
Up the difficulty, run around and use all the different weapons. The spider bait (cant remember the name of it now) was always one of my Fav's along with the gravity gun.
They're all just legendry games made for PC
i guess. That's why to enjoy them these days you need to have an open mind to how old the series actually is now.....