Warhammer 40,000: Dawn of War + the first two expansion packs (The Dark Crusade especially).

It probably remains my favourite RTS of all time - it borrows a lot from Starcraft and Warcraft, but I think it just does everything better. Brilliant, addictive gameplay, huge campaigns, with near infinite replay value in Dark Crusade, still a fairly good online community, great units, 7 playable races (if you do the sensible thing and avoid the third expansion Soulstorm which was a hurried mess and broke the game), and some of the most inventive & bloodiest kill animations you'll ever see in any game. The only game I've spent more time playing in the past 5 years than WoW, and I'm still in love.

Others have mentioned Company of Heroes, made by the same company, which is also incredible. It's just not got as fun a setting as DoW, though the online is probably a little better. Dawn of War 2 isn't anywhere near their level in my opinion, unfortunately, but has a very active community so it might be for you.