Depends, do you mean certain games or the entire series?
C&C was just groundbreaking for its time in just about every possible way - it was one of those classics that had everything -and even today still has the best RTS storyline I have seen around, but its sequels did not quite live up to those standards. TA is simply an extremely well-polished and overall solid game - this is the kind of thing I really like to see, more so than originality - and of course, the mods out there for TA are incredible. Out of those two, it's hard to say really; in this day I would usually play TA due to its newer technology and gameplay (by the time it came out, RTS was firmly established), but cannot count all the hours I spent with C&C up until only a few years ago.