Zero-K. It’s a competitive online RTS loosely based on the classic Total Annihilation (which led to Planetary Annihilation and Supreme Commander). Some of its features are a bit overcomplicated, but it does an amazing job innovating within the RTS genre.
It’s fast, aggressive, and fun. You spam units, claim territory (in the form of metal-extractors and energy-grid that upgrades their output, and building defenses to protect that) and raid and assault your opponents.
It abandons the hoary old concept of factions, instead giving you your choice of starting factories… and as the game progresses, you can expand into other factories to access the synergy of units. So you start a battle with a narrow slice of the unit-pie, but ultimately can access the whole inventory in a single match. There is no “teching” really, besides constructing resource-buildings, which keeps the focus on resources, construction, and combat.
It has a full single-player campaign that introduces the game’s complexity bit-by-bit… but the campaign does have some difficulty spikes, particularly since the units do get rebalanced once in a while and so an old mission will become suddenly easier or harder as the developers patch the game.
The game has a Lua-based GUI plug-in architecture if you like WoW-style UI mods as well.