timppu: Sunken Colony in Starcraft. There's just something very pleasing with that defensive structure hitting unsuspecting enemies from ground up, squish!
I almost mentioned
Lurker instead which is a bit similar but even better, but then remembered it is an unit, not a structure. Still it feels like a portable defensive structure...
Oh gosh, the Sunken Colonies always creeped me out when they hit me, thankfully I was playing Zerg myself most of the time and Lurkers were my favourite SC units in Broodwar too.
Oh how I wish that someone would polish one of the existing HD hacks for Starcraft: Broodwar.
timppu: In general I like the Zerg structures as they keep expanding that gooey stuff on the ground. Similarly, I enjoy fighting the zergs, destroying their buildings so that that gooey stuff goes away. Fights where zergs are not involved (only humans and/or protoss) are less interesting to me.
The undead have a similar mechanic in Warcraft 3, it's quite satisfying indeed, must have something to do with conquering landmarks.