got C&C Generals: Zero Hour here. :) thats certainly a good RTS series. that generals challenge in zero hour gets quite lengthy after a while though. you pick one general to play as (basically choosing your specialty...i typically go for the tank general and those emperor tanks. lol bring on the slow yet VERY powerful tanks.) then fight through the rest of them all leading up to the 5-star Tibetan general wielding all the powers of the lesser generals. man, it can take over 3-5 hours on that one fight alone but it sure is fun.

another game you might like is AI War by Arcen Games

warning, AI War can literally make your computer weep if left unchecked. a single campaign on easiest difficulty can also easily last you a week or so. lol you can easily reach over 10k units across the universe map eventually if things get dragged out long enough. especially if you come across this one AI building. i don't remember it's name but if you get near it and attack and/or try hacking the building it'll start spawning non-stop waves of AI controlled ships with each wave stronger than the last. if that building isn't destroyed immediately upon being discovered it can easily wind up spawning enough units to crash the game. iirc that even happened to one of the devs computers one time so beware.