Tripped over this topic in my other thread just now about Cannon Fodder. Be they assassins, jet fighters, wizards or nukes, which combat unit(s) live on in your memory long after the game is complete?
My AT above is one of a line of similar units from the Command & Conquer series—it really is an army in a can. I used to often base a whole mission strategy around it.
1. Get a couple, one each approach to your base beside a built—and therefore repairable!—defense structure, and you're safe.
2. A squad of 4-6 can almost always own their map sector, self-repair meaning they don't have to return to base. Keep pulling them just behind each other when damaged from concentrated attack, and let the enemy waste themselves in your gauntlet.
3. Great on open maps with few choke points—they're unstoppable only when they can support each other side-by-side. Much less useful in closed terrain.
My other favorite would be an air unit—I always go for air power when available. Something like the Hind Chopper, which is excellent to target any anti-air support infantry so other air units can tackle the armor. Also a major PITA for any isolated unit like a Harvester, as it can stay on attack for a long time before needing to return to reload.
Or the Mig Fighter from C&C 1, excellent at the primary job—hit fast and accurately, and escape quickly. I always felt a lot safer once I got ~4 of them built, no surprise enemy incursion would be unattended for long. Their reload time was really quick too, so you could keep an assault cycle going.