I have not played earth 2160 in some time, so have that in mind.
Overall, I would recommend total annihilation. TA is considered to be one of the all time greatest RTSs. It has plenty of different units, and allows for a lot of different strategies. It is also a rather challenging game, with a high skill ceiling. You also get a lot of game for your money with TA, because not only do you get a lengthy campaign, you also get the expansion and all the bonus units, which expands the game even further.
Another game that I would recommend checking out is
Warzone 2100 (but do get TA as well). Warzone offers quite a few interesting gameplay elements, like persistent units, unit design and some rather interesting unit abilities. It is one of my all time favourite RTSs (and it is free!)