Armored Core is one of the best things to come out of Japan since Squaresoft, and it's the only good game Agetec has ever done (Eternal Ring, anyone?).
The premise is simple. You have a 'mech. You fight with it. Stuff blows up. Then it gets good.
You see, there's hundreds of different parts to customize your 'mech with, purchasable with the money you earn by completing missions, as well as scrapping enemy 'mechs in the arena.
The level of depth in the customization process is whats amazing. There is no 'best part' for any given section. You have to take weight into consideration, as well as power usage, ammo, etc., etc. and decide what type of 'mech you want to build.
Some people prefer tanks, with lumberingly slow tread legs that can carry a ton or twelve of blisteringly powerful weaponry. Some people prefer a stealth 'mech that doesn't show up on enemy radars and can either ping with a machine gun from afar, or sneak up and backstab with a laser sword. Still others prefer a light, speedy 'mech that can dash around the arena, spewing homing missiles to ruin the other guys day. And that's just the beginning of the endless possibilities that any one of the games in the series offers.
Rock.