I'll rank them:
1) Half Life 2 (and the Episodes)
2) Portal 1 and 2
3) Half Life (and the expansions)
4) Team Fortress Classic
And the rest. Some of which are alright but I have no strong feelings about (TF2, Day of Defeat, Left 4 Dead). Some of which I actively dislike (everything related to Counter Strike, Left 4 Dead 2).
My ranking is a bit off, though. In terms of "fondest memories", the original Half Life and TFC definitely rank the highest. Those games represent the golden age of PC gaming to me. HL2 is objectively a better game, and it definitely blew me away more. But, it just can't compare to the memories I had with the original.
I should address Team Fortress as well. TF2 is an objectively great game. It's funny, well balanced, and polished. But, the magic of the original is totally lost for me. The original had a sort of unintentional ridiculousness about it. Weird exploits they didn't bother fixing, like rocket jumping, concussion grenade jumping, shooting team mates across the map as a sniper, bits of the map that you could force your way through and wreak havoc, the unintentional goofiness of the original character models (the HW guy was basically a care bear!), the ridiculous distorted physics, messing with the server commands and further distorting the physics, the way players gibbed if you shot them just right, and how they exploded into a big mess of turkey drum sticks and multiple skulls (more so if you cranked cl_gibcount in the console). The fact that they embraced all that for TF2 is simultaneously awesome, but somehow ruined it for me. And, dammit I miss grenades!