My top 5:
1. DoD_orange_indoor_arena: Always have kdr of 5 there, like 300 kills, 60 deaths xD
2. DoD_Mario_air: Great sniper map
3. Avalanche: Best server we have
4. DoD_pooldayQQ: Great hideouts
5. Donner: Great practise
In beta 1.3b days, Caen all the way. Since retail, Avalanche, especially in Source. I used to hate Donner, but I'm starting to like it more and more. Ultimately, all the Source maps are pretty good, IMHO.