While many people have played or at least heard of Tribes, the game that series is tenuously based off of is comparitively unknown. Starsiege is my favorite game in the mech genre.
Starsiege is the sequel to Earthsiege and Earthsiege 2. It continues (and retcons) the story of mankind creating a fully sentient AI named Prometheus, and eventually an army of AI controlled HERCULANS (the game's term for mechs) called Cybrids. Eventually Prometheus decides that serving humanity really isn't where it's at, fucks up Earth a bunch and flees to the outskirts of the Solar System to brood.
The first chunk of the game takes place far enough after this to where people forget about the Cybrid threat, and instead focus and killing each other instead. You control a generic colonist on Mars who, along with the rest of the planet, is being brutally oppressed by Earth. The emperor of Earth, being the bejiggidy ass that he is decides that Mars isn't pulling its fair share of protecting against the Cybrid threat everyone's forgot about. After a couple hours dicking around with the Imperial forces, the Cybrids unsuprisingly start coming to town to kill everyone.
My favorite part of the game, by far is how much you can customize your mech. While it may not have the sheer number of parts the Armored Core series might have, Starsiege makes up for it by not having the vast majority of parts being useless. Most parts and weapons in the game are situationally useful, and the game's campaign is varied enough to make tinkering with your mech a pretty important skill to pick up.
Multiplayer is an area where the game really used to shine. I have no clue what the community is like now (I lost my disc when I moved), but back in the day this was my favorite online game. If a team had proper communication, it almost played like a proper class-based FPS (but much, much slower). There's also deathmatch, but it doesn't ever feel too spammy-- usually maps are fairly large, and the server limit was around 20, IIRC.
Running this game may be problematic on a modern system-- the last time I tried to install it hardware rendering had weird graphical glitching everywhere and was near-unplayable. And software rendering is ridiculously ugly. Also, to get it running online, I'm fairly sure you need to download a community-made patch for the master server list, but that's not too strange for a game more than a decade old.