Whoa, this topic is ridiculously long.
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I read through some of it, used search button a bit and apparently there is one serious contender that has not been mentioned so far. Total Annihilation.
Even if it lacks starships entirely at the time of game's campaign (because of them being too energy inefficient), very few fictional factions could ever hope to compete with Arm or Core in galactic scale warfare. Industrial capability of TA units in a prolonged conflict would most likely be too much for any of the factions included in the poll; a single unit being capable of reproducing exponentially, turning an entire planet's surface (and crust and mantle) into a military complex in matter of days. Any noteworthy technologies used by opposing forces could be taken over with viral nanobots and replicated. All mass of all fallen combat units would be reclaimed, recycled and sent to wage war again and again in new forms.
All TA units are gravity shielded to some degree (or have anti-gravity suite in case of aerial units) and all feature so called "weak cloaking" which makes them nigh invisible to naked eye and basic sensors, and some have more advanced forms of cloaking and other such electroric warfare. Apart from sensors of Star Trek, their presence could go unnoticed until their numbers become unstoppable. There are no energy shields in TA, but armor is forcefield enhanced so that entire exterior of each unit is very dense and behaves like a single super molecule. "Heavy armor" tends to fail catastrophically if generators are damaged though.
Discounting the implosion device that would "reboot" the entire galaxy, TA only has weapon systems in scale of what can be fit to tanks and kbots. Then again, projectiles are built into barrels by spraying nanobots and antimatter is regularly used for both payloads and energy storage (and freely shared between all units via short range teleporter network.) Spheres of self-containing forcefields packed with dense degenerate plasma and multiple seconds long laser beams "frozen" in tiny grid of space time get shot around just as often. And then there are disintegrator guns that suppress gluons and let their targets rip themselves apart on sub-atomic level.
Main drawback of Arm and Core would be lack of means to quickly leave a system if a need arises. They do have long distance teleportation devices called "galactic gates" which can transport unit(s) instantaneously over distances typically up to 10 lightyears, but they require days or possibly weeks to charge up, and even then only something like several hundred tons of mass can be transported at most.