Zero-K is pretty good, if a bit obscure, with a relatively small community.
Another TA derived game, like Supcom or Planetary Annihilation. It's got a wide variety of units, though some can be quite gimmicky. It does away with tiers and factions in favour of a more resource-centric progression and differentiation system. It's got a decent AI, and they're working on an improved version of it that will be released at some point. The interface is quite powerful, giving you a lot of options for how you select, control, or build your units. Unfortunately, it does a pretty poor job of actually teaching the player about these options, so using the documentation and help from the community are important. The combat is focused on counters, with unit composition and positioning playing a much bigger role than raw numbers. There's also a terraforming mechanic that adds another layer to it, though it's best used sparingly. The campaign is a slog, though, as the game is very multilayer focused.
Overall, it's a fun game with a lot of depth and consistent updates, and I highly recommend it.