To be honest, anything you like - although I'd prefer if it was an official design and if it's using any exotic tech I'd appreciate it if you brought the appropriate rules along, although I will have my copies of tac-ops and interplanetary ops available.
However, I'd suggest bringing a "stable" of four or more mechs, one in each weight class and using relatively conventional tech. The event is meant to be for beginners as well as veterans so be nice and only bring one horribly abusive machine... ;)
I'm running the event so I'll be bringing a butt-ton on mechs of different weights and tech levels for people to pick up and play, so I should be able to match something reasonably fair against anything that a player brings themselves.