Well, it depends on the type of course (hilly/flat, straight/tricky, short/long, etc) and what form they're in. To get a feel, check out the top10 in the Olmpic TT, World Championship TT and the TTs in TdF, Giro and Vuelta.
I would say if everybody "peaked" top 5 could be
1 Fabian Cancellara
2 Tony Martin
3 David Millar
4 Alberto Contador
5 Gustav Larsson
Maybe Contador could be higher if that was his primary goal.