omg dod_avalanche.. pure genius.
Takes the real Valve guys to make something this good, and I'm glad they chipped on.
Truely a work of art, visuals match gameplay.
Same story for the Valve maps on CS, they are VERY well done.
Chateau, Piranesi (Dont really like the Havana one.. too dark or something)
But, I think they have too many routes, and the average CS player can't handle too much thought at once, so they werent very popular with the community.
Theres a one official Q3 map that I love also, lots of levels and the flow is great. Cant remember the name though.
Being a mapper myself though, I feel this thread is pretty 'open' as you didnt really specify what makes a 'best map' be it looks, or gameplay, or both, etc.