Heroes of might and magic.
I - is here on site
II and III are good but still sold i believe
V seems also good
They are not RTS, but combat is fast enough to keep you occupied.
Out of Sci-Fi (turn based):
Galactic civilisations II is OK
Space empires III is also OK, while SE IV is a bit too complex IMO.
RTS in older days - i would say Settlers III and Settlers IV are good. Sort of Age of empires only much larger armies (like 200 vs. 200
)