I probably have more hours on the Source version now, although I haven't played properly for over year (closet TF2 addict).
For nostalgia: <1.6.
I remember all the hastle/fun I had with patching it up to the latest version and the championing PC Gamer used to give it. Many happy hours on an internet connection that seems positively dark-aged by todays standards.
Fiddling with skins, creating bot waypoints, custom-everything
I have a confesion though -
cs_assault was my favourite map...
I'll get my coat.