fy_iceworld was my favorite from 1.6. I recall nothing could touch it with regard to rapid, repeatable chaos. I've probably put 500 hours into that one map alone. You will find an almost identical structure in virtually every Call of Duty multiplayer experience - One big room with 4 "cubes" in it that players fight around, with various degrees of partial cover scattered throughout.
Don't forget fy_pool_day! I always found it such a shame that a map like as_oilrig never took off in a competitive sense. Always thought "protect the VIP" was a really cool concept, but I guess the rest of the world disagreed.
Agreed on the hostage maps. TFC had that one map too...