Survival Mode on No Mercy. Seriously, this mode was fucking hard. You had to eliminate (at the most) 101 people. Eventually, you'll get so worn down, that a weak slap from Mae Young could send you over the top rope.
It's the quickest way to unlock everything though. I remember playing it at 3am when I was 12, and HBK came out after 80 eliminations. I freaked the fuck out, waking my dad up in the process. GOAT game.