Tactical Nukes.
But if I don't pick the obvious, then artillery. Sure, rifles/cuir/cavalry are more efficient, but aside from nukes there is nothing more comical than the trashy/can't ever truly defend it/lollerarty. Even end-game units struggle against CR III + CR II (follow-up) arty. Only end-of-tree gunships can flank it (and SAMS which come much earlier hose them). With high CR it blows through tanks, infantry, marines, machine guns, and doesn't even struggle much against mech infantry. It bombards defenses nigh instantly no matter what. It has enough collateral and strength to be a serious threat to stacks in the field too. If the enemy spams tanks, the tech unlocking it also opens up anti-tanks to work into stacks.
Obviously, it doesn't have the insta-kill, gift-trolling (give one to each AI near its opponent's stack and watch as neither side has any units and they both hate each other greatly, while not minding that you gifted the missiles!) stack erasing power of tactical nukes, which are hands-down the most lulzy unit in the game. Still, if you want to get away from the "tech to renaissance and abuse the window" playstyle, few things can match it.