Armored core nexus introduced dual stick support while the older ones require modding in order to avoid the pain of severely outdated control schemes. The pre-ps3 ones can be a bit aggressively anti-player with dark souls style instant deaths, but they’re still fantastic in their own right. Just skip armored core 5, it’s basically a multiplayer demo and its sequel does everything immeasurably better.

The old ones have some clunky ass cam controls. That's the most aged part of the experience. Customizing and experimenting with your AC has always been an awesome experience since the first game, though.