Tiberium Sun would have to be the game out of the series with the most hours put in from me, most of which were online back in the days when it was active. Some of those mod maps with extreme amounts of money and base walls took forever. NOD with their stealth and artilleray had a clear advantage if they weren't stomped quickly.
I've been playing C&C since the first release though. Never eneded up buying the original Red Alert but I borrowed it from the video store that many times and played it at a mates house. I did however get RA: Retaliation which was pretty sweet. I also have all of the other titles, but not their expansions except for Firestorm.
Looking forward to RA3, the bears look like an addition awesome considering that Stalin experimented with a lot of that sort of stuff.