C&C 3 is great if you ask me. They had a pretty tough task releasing a game living up to the originals, with an all new studio, so many years later but I think it's very well done.
I haven't played any after that, but the C&C3 expansion still got decent reviews, but not as good as C&C3.
Red Alert 3 also got quite good reviews, almost as good as C&C3. I loved C&C and Red Alert games, so I've been thinking about picking up RA3. The biggest complaint I heard is corny cut scenes but c'mon that's not a surprise and who cares if it's fun. RA3 expansion sounds sort of like the C&C3 expansion - okay, but certainly not better than the regular game.
As for C&C4, it got slammed pretty hard in the reviews. Apparently it's changed quite a bit of the regular C&C gameplay.