I would have to say Castle or the map I can't remember the name of in the 3rd Map Pack that was on a large street with broken down buildings on each side. and giant gate in the middle.
All the maps other than the map pack maps (wasn't a big fan of the majority of them). I really liked the w@w maps (even though I didn't like the general multiplayer experience on that game due to Mp40's and Juggernaut). I hope they do bring some of the w@w maps back in the map packs and possibly even some of the cod4 maps (thought they would need to add a bit of a twist to them to make them feel "cold war-ish").