I don't really consider Settler's two to be a RTS game because you really don't build or do a lot of stuff in real time. Not like you directly control the villagers. I prefer TTS in some games such as Heroes of Might and magic but I prefer RTS in Age of Empires.
RTS: Warcraft I - Not Beaten Warcraft II - Beaten Warcraft III: Reign Of Chaos - Not Yet.. Command & Conquer - Beaten Command & Conquer: Tiberian Sun - Beaten Red Alert - Beaten Red Alert 2 (+Yuri's Revenge) - Beaten Age Of Empires (+Rise Of Rome) - Beaten Age Of Empires 2 - Beaten Starcraft - Beaten Empire Earth - Partly Beaten Z - Beaten Warrior Kings - Not Beaten Battle Realms - Not Beaten Sacrifice - Not Beaten
TTS: The Settlers - Not Beaten Settlers II - Not Beaten Settlers III - Not Beaten Settlers IV - Not Beaten M.A.X 2 - Not Beaten Civilization II - I think I've beaten it.. Civilization III - Not Beaten Heroes Of Might and Magic 2 - Partly Beaten Heroes Of Might And Magic 3 (+ Armageddons Blade) - Beaten Heroes Of Might and Magic 4 - Partly Beaten
Heroes IV is more fun, but I had more fun playing Heroes III, although it was quite a while now.
In the RTS, I'd choose Red Alert, that one would probably win in the gametime thread. It was about the only game I played for three years. I'm eager for the next part of C&C, if there will be any..
Oh, and some more TTS: Ceasar - Not Beaten Ceasar II - Not Beaten Ceasar III - Not Beaten Pharao - Not Beaten