There are so many good tactical games out there. I always enjoying play SRW series. you just can't get enough mecha vs mecha. My favorite out of the series is SRW MX. Love the story, gameplay, cut scenes and soundtrack.
As for C&C, I love all the games. However, I enjoy Red Alert 2 the most. Tell you the truth, I was a little disappoint when Westwood announced C&C Tiberium War... I was hoping to be Red Alert 3.
Not sure this count as tactical game, but I enjoy playing Total War series as well.
I know you ask us to pick just one... but there are so many good tactical games out there, is hard to pick just one.