Based on appearance only, Hector Lombard, Thiago Silva or Evangelista "Cyborg" Santos. But Hector is probably the most intimidating fighter in the UFC. He'd kill me in 30 seconds, maybe less.
EDIT: after reading the whole thread, I'm under the impression some posters here would actually love to run into some of these big, muscular guys in a dark alleyway... without anybody looking.