Impossible to decide: all 3 subseries had a good and fun game
Tiberian Sun + Firestorm has the best story Red Alert 2 + Yuri's Revenge has the most fun gameplay Zero Hour's idea of subfactions were better implemented and balanced, and hence is far superior to the "subfactions" in Kane's Wrath. I could quite literally play the same way for each KW subfactions while I cant do that with Zero Hour's one.
Dr Thrax FTW
Action speaks louder than words. But words, when used right, overwhelm any action - Me, 2006 Let's put a smile on that face - The Joker, 2008
I loved Red Alert but I think I spent the most time playing the Generals/Zero Hour games. I would have loved it if they did a decent sequel but EA can't think of how to put microtransactions in the normal game so they can't do it (thank God!).