I'm going to recommend
Sins of a Solar Empire, of course, which claims to redefine the genre as being a 4x-RTS. Sins has a slick and crisp feel to it and was designed to try to reduce or eliminate the importance of frantic clicking and minute micromanagement while retaining the strategic and tactical elements of other RTS games while also adding some 4x elements. It offers 3 races, an extensive tech tree, and support for up to 10 people in online multiplayer. This game is full of strategic considerations--you really need to think about everything you do, from where a structure should be placed, what planets to colonize, what tech to research, whether to expand your economy for a long-term benefit or whether to build a bigger fleet now for the short-term, and how many and which types of ships to build with your limited resources.
The real Sins game is, of course, online multiplayer against human opponents, which is very challenging and intense, especially 5v5's with balanced teams and experienced players. (These are played on the Huge Random Single Star map.) (Most 5v5's take about 1.5-2 hours, so if you can ever find time to watch a movie you can play 5v5's online.) (Do note that most of the pros and 4v4 and 5v5 games are being played on the $10 Entrenchment expansion, which enhanced the game.)
Chart-box just named Sins as the 4th best RTS of all time, behind Homeworld, C&C Red Alert 2, and Starcraft.
http://www.chart-box.com/charts/best-real-time-stratergy-games
Here is a decent video review:
http://www.youtube.com/watch?v=xY-NSNhgydY