If its the same as last year I'd say a Cyclocross bike. Boring boring track, just climbing in fields. There was one good techincal descent that they took out when the roadies moaned.
EDIT Sorry having been that negative I should add that I did enjoy the event overall, was riding bikes afterall and the night laps were lovely. I'd say hardtail is defo the way to go but you can ride around on anything as long as you are prepared for a lot of climbing.