I'd go for Half-Life Source. It's Half-Life updated to use the Source engine, though without any enhancements to the actual game content. The original Quake-based engine is pretty rough on the eyes today, unless you're running it on an old 3dfx rig or something.

I've never been able to get into Blue Shift, or Opposing Force for that matter. Even HL1 was a bit of a chore to play through. I did finish it before playing HL2, but you can skip over it if you want. The Half-Life story feels more like a prequel to HL2 than the first game in a series, at this point.