As far as real time strategy games go, I think I've found my absoulte favorite.  DOTA or Defence of the ancients is a custom Warcraft 3 map where 10 players - 5 on each opposing team use troops (referred to as 'creeps') to get to the opposing teams main base and destroy it.

After years of gaming and playing things like Team Fortress, Left 4 Dead, Counterstrike, Bioshock, and even more obscure games like Gunbound, I find myself always coming back to DOTA to because the strategy of the game never gets boring.  

 Now the makers of the custom map are coming out with thier own game!!!

It's called Heroes of Newerth and it looks fantastic.  I encourage anyone who used to /does play DOTA to check it out.  And if you didn't play or havn't heard of it when the open beta starts it'd be a good time to check it out.  signup on their webpage and just check out the game here:

 http://www.heroesofnewerth.com/