Whenever I think "Hey, wouldnt it be cool of there was a map like this?" I build it. So pretty much the maps I enjoy playing the most are...well...my own.
As far as maps by other people, I'd say Kindgom Purge and Monolith (not the blizz one) and Blizz's Azeroth Grand Prix are my favorite custom maps. -VGsatomi
Legend of Magnador(none blizz) and Azeroth Grand Prix (blizz). Azeroth Grand Prix rules!. It's model, trigger, multiboard, gameplay, terrain, system, and cinematic are rocks!
actually there arent really good multiplayer maps (i h8 tds and the rest i dont really like). still my favorite are:
1º eve of the apocalypse (aka Eota) 2º battleships 3º ninja vs samurai 4º dbz tribute ORIGINAL (i dont play this one long time ago, as everyone only play that dumb edited version) 5º dota ORIGINAL (which i dont play since RoC)
btw "Blizz's Azeroth Grand Prix" is good too, but in multi it usually lags as hell and because of that it isnt in my list