Really most of them are pretty simple at their core, but it's like chess: simple to learn, hard to master. Here are some suggestions to get you started: Warcraft 3 Heroes of Might and Magic 3 Civilization V Starcraft 2 --- -What is best in life? -To crush your enemies, see them driven before you, and to hear the lamentation of the women.
Command and Conquer. They're as simple as you can get, since it's focused entirely on combat and there's only one resource, so you don't have to worry about micromanaging an economy or diplomacy or any of those distractions, and it's a ton of fun.
As for which one, they're all great, apart from number 4. Generals and the Zero Hour expansion (necessary) are fantastic. So is Red Alert 2 + Yuri's Revenge (3 is alright, but doesn't have the same charm). Command and Conquer 3 is also a very good game. It's well balanced, big budget, and has a ton of content.
I'm also very fond of Total Annihilation (older game) and Supreme Commander (newer game from the same team). Those two are very different from any other RTSes in terms of their mechanics.
I wouldn't recommend Civilizations. It's very complicated and games can take forever. --- Some say that his voice can only be heard by cats, and that he has two sets of knees... all we know is, he's called the Stig.
Dawn of War is very fun and simple. Pretty easy too. It's a really good starter RTS. --- My fight stick: http://img703.imageshack.us/img703/2687/sdc10110k.jpg My team combos: http://www.youtube.com/watch?v=RdoD7BvGUOE
Really most of them are pretty simple at their core, but it's like chess: simple to learn, hard to master. Here are some suggestions to get you started: Warcraft 3 Heroes of Might and Magic 3 Civilization V Starcraft 2
Just FYI Civ 5 is a TBS not a RTS... --- "The two most common elements in the universe are Hydrogen and stupidity." Harlan Ellison