.

Age of Empires II.
Red Alert II.
Empire Earth.

Edit: Ninjas.

Starcraft, Age of Empires 2, Rise of Nations. If you like the classics.

Beaten so hard on Red Alert 2, but... Red Alert 2.

Warcraft!

C&C Generals / Zero Hour

AoE 2,3

Lord of the rings BFME 1 and 2

starcraft and warcraft 3 are the best in my opinion ;)

Starcraft or GTFO

I believe Rome Total War has a lan option.

Battle Realms! Was quite nice :p

Look up a service called Tunngle. It allows you to play LAN games with friends over the 'net, and you can still keep it private like a typical LAN (ie. it doesn't have to be some publicly advertised server). Pretty cool. They're still in beta right now, but that works in our favor, since during this period you can get a lifetime premium account for one cheapo payment (something like ten bucks --I forget what the conversion was when I did it, but it's right around there).

C&C 3? You know you want those mammoths.

Star Wars: Empire at War

Supreme Commander FA w/ sorian ai

Stronghold Crusadere

The only correct answer is Warcraft 3.

Large scale: Supreme commander
Small scale: Company of heroes

Starcraft 2 has no LAN mode, just online mode. But if you have a good internet connection on your LAN party, this should be no problem for a few people. I think it's the best RTS at the moment.

Age of Empires 2

Also I keep hearing good things about Moonbase Commander, I think I'll pick it up at some point. It's turn based though.

Starcraft, Warcraft III and Company of Heroes

When I hear RTS the first things that come to mind are, Starcraft, Company of Heroes and Command and Conquer

Age of empires 2 is a classic for me. Maybe Some civ 4/5GK for Turn based?