-=Oligarchs=-
Land - Tanks for every occasion, purpose built.
Mech - Exo-suited infantry, slower but longer range than land
Sea - Primarily battleships and naval superiority
Air - Air superiority fighters, scout ships, and a few fighter-bombers.
Space - Dedicated space facility, the highest technology, the strongest units.

-=Rebels=-
Drone - Inferior line of longer-range, slower firing tanks. Drone operated means no population limit though.
Adv. Mech - Multi-purpose mechs
Adv. Sea - Floating artillery for bombarding land and submarines for hunting battleships.
Aerospace - Limited air options, primarily drop-ships and carriers, branches into lower tech space units and space-planes.
Experimentals - Lacking proper space construction you get super-tanks and walking-fortresses instead. As well as dedicated "surface to orbital" cannons for the enemy space units.

[note: all other factions tech up to T3 with their final Space/Experimantal being T4]

-=Sentient Aliens=-
Nest - Produces bugs needed to start the terraformation process, required for T0 and T1 (falls off T2)
Mech - Chaff general unit production, very high defense. The frontline for your actual army.
Adv. Land -Rover production, faster than tanks, better guns, weaker armor, backbone of your early armies.
Hatchery - Bug production, weaker than your rovers but all with niche abilities like berserk, burrow, chain-lightning, spawning MORE bugs just by living, etc.
[everything above caps at T2 and won't see any use in the late game]
Aerospace - The real backbone of your army, spacecraft that's atmospheric capable to dominate air superiority.
Experimental - Four highly specialized ground units to suppliment your space force. Mainly anti-anti-air and anti-anti-space units (go figure.)
Spacecraft -The meat and potatoes, the final frontier, the "I win button", these units reach T6 limited only by a branching tech tree. (Centuari - The stronger and more specialized star trek styled aliens. or Narn - the cheaper, more numerous, and star wars inspired aliens.)