This is an excellent question. There is a difference between choosing a great RTS for personal play and an RTS for a LAN party. Personally, I have hosted multiple LAN parties and found two games in particular to be exceptional for team playing due to their custom map enhancements.

The Blizzard games World of Warcraft 3 and Starcraft 2 will offer you the most versatility and bang for your buck. There are literally thousands of custom maps that can be downloaded through the Battle.net service that are basically games themselves developed by other gamers. These maps may be tower defense games, RPGs, strategy games, action or arcade games. They basically use the custom map engine provided with the game by Blizzard to develop their own games.

Blizzard has done this with their RTS games since the original Starcraft over 10 years ago, so they have perfected the technology, and users who develop maps have done an outstanding job of creating solid stand-alone game maps that offer extreme versatility to the RTS genre.

Hands down I would pick up one of the two, you will have much more bang for your buck if you stick with Warcraft 3 because you can get the box set for a reasonable price now that Starcraft 2 is out. Of course, Starcraft 2 looks much better and has more advanced features.

There are also a number of maps that offer co-operative play. Lets not forget the actual versus mode available in all RTS games is also available for these as well. You can find the most popular custom Starcraft 2 maps here. You can find the most popular custom Warcraft 3 maps here.

As for the other RTS games, I really enjoy them all. I played Aged of Empire, Empire Earth and Supreme Commander but wouldn't really find much interest in playing those with others in a LAN environment. I actually never really liked the command and conquer series.