My first thought would be Doomsday. Unstoppable, ugly beast that just wants to maim and destroy... Nothing on earth can stop him... even killed superman... That's a scary monster.
Joker? he's crazy... he's a killer... but he's still just a dude that looks like a clown. There is just no legitimate reason why any random person can't bash his head in with a crowbar. Despite his craziness... he's just WAY too mortal for the hype he has.
Grodd would be pretty scary too... especially if he's in 'rip you apart and then actually EAT you' mode...