I was never into pc gaming until HL2, only ps2. I had just bought a laptop (in 2004?) and decided to get the best fps that was out - i bought HL2 and played it to the end. I really enjoyed it, the atmosphere, the weapons, the sound. I bought HL and the other HL games (blue shift, opposing force) but only played HL maybe 1/2 way through, after i had finished HL2. I played HL2 ep1 to the end (but got stuck at the end) and am stuck at the end of HL2 ep2. The only other single player fps that has got me interested since then is bioshock.
As for the multiplayers I keep coming back to DODS, not battlefield (1942 or vietnam or 2 or whatever), or COD (1, 2, 3, 4, 5, 6, 7, 8 etc), or CSS, or UT2k4/UT3. But maybe that is about the TOG DODS division more than anything else.
My guess is that HL1 is more history than anything. Play it to see what the evolution of gaming was about. And Blue Shift and Opposing force are about sequels and mods and integrated storylines. And HL2 was a serious step forward with story and playability and graphics.