I'm about three quarters of the way through half life 2 having only invested in a decent PC a few weeks ago...
Half LIfe 1 was and remains one of my favourite shooters of all time. I played it on first release and for the time it was a million miles ahead of anything else that had ever been produced. The no cutscene style makes the character so much more real and drags you right into his story. It also had groundbreaking level design (not just chucking X number of monsters at you, but turning every battle into a set piece) and phenomenal AI for the time. Some of the battles against the soldiers in that game were legitimately hard just because they seemed to know exactly what you were going to do and worked as a team against you.
Now Half Life 2 isn't leaving me with quite the same impression, though it isn't far off. I feel it is a little more repetitive- no longer is every battle unique, and the levels late in the game are getting to be a bit of a drag. The driving sequences were also MUCH too long. Story wise I feel it is better, with the setting much more appealing, the graphics are incredible for the time, and once again I find myself immersed in the life of Gordon Freeman.
Best first person series ever? It has to be close. Certainly it has a consistency that few of the others can match, and still does things that no other series does, whilst innovating a bunch of others. No, it isn't the full blown action every second that COD is, but sometimes a more cerebral and intteresting game can trump that.