I would pick 2fort if I had to pick one. I spent the most hours there and had so much fun.
There's a few great surf maps but I can't remember them off the top of my head.
I'll give a good mention to Blackmesa of course - I used to wake up on Sunday mornings to fill it with a friend who was several hours ahead of my time zone when it was in it's prime as a 24/7 server.
And one more shoutout to the "PrideRock" Lion King map... only because we had a hilarious event there once that I had a ton of fun on.