Obelisk of NOD and Tesla Coil were always really great to build. The superweapons were also really fun to use in C&C games. Using the Chronosphere to teleport nine Prism Tanks inside the enemy base was always a wonderful way to ruin your opponent's day. :D

I like anything that helps me turtle.

Starbase in Sins of a Solar Empire
necron monolith in Dawn of War that spawn free units that can be resurrected or their corpse converted to stronger units with a Spyder
Graveyard and necromancer to spawn a sea of free units for Warcraft
Siege tank in Starcraft
The many long range cannon / lasers in supcom

Great! A DoW player! Personally I like the look of Imperial Guard bases in DoW, with their more traditional military look, but I think that Tau takes the cake when it comes to cool base buildings. My favorite building in DoW though is probably the Space Marine orbital relay. It doesn't look like much granted, but It does enable you to fire dreadnaughts straight into enemy ranks which never gets old!

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...

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.

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.

The undead have a similar mechanic in Warcraft 3, it's quite satisfying indeed, must have something to do with conquering landmarks.

Same here. I never had a similar rush with other rts.

Tesla Coil Love Song

I have to say it's Men of War.

Probably the ogre mound, just because of that funny nom-nom-nom sound it made when you clicked on it. I always loved that sound effect and would click on the ogre mound sometimes just to hear it. Actually lots of buildings and units in the Warcraft games had great sound design.

Ugh yes I remember that sound. Kinda freaked me out :P I preferred the gnomish inventor. The building made a funny whistling sound when clicked.

Amazingly, this site has a lot of the sounds: http://www.thanatosrealms.com/war2/alliance-sounds

I also like the imperial Guards millions guns tactic or the tau that fire the enemy while the enemy have to run a long way to fire back. But I guess that is not building so I did not brought it up earlier. (I consider siege tank buildings when mounted).
https://i.ytimg.com/vi/9MSZBfeKnwE/maxresdefault.jpg

It gives a satisfying feeling when you can press your advantage against the enemies weakness.

http://www.dealspwn.com/writer/wordpress/wp-content/uploads/2012/04/Star-Trek-Armada-II-4.jpg

Remember that?

yeah that was some scary shit.

Why those, they take all the fun out of the game. :P

Just plant it and forget it. XD

The arcologies not the coils.

The ultimate in urban renewal. Just launch the fucker into space and start fresh.

I think those imperial guard soldiers and tanks might be wasting their ammo on a Nightbringer by the looks of it :)