600pp is the range of pp that you can almost win with every car that can enter.
A few cars depend on the track as they either need more speed than they can muster, or propably win by fuel consumption/tyre wear rate.
For example have "we" done the Tokyo 600 event with cars that were down to 530pp, or cars with a lead of more than 2 minutes, or cars with barely 250hp, it really is winnable with the biggest selection, but depending on each track and propably the setup.
Pick Ford Focus RS with AWD and tune it up, its generally well rounded.
Pick Audi R8, its generally faster but sips more fuel.
Pick Alfa Gulia and do just a slight adjustment, faster but less handling.
Pick Toyota TR030 and downtune and drive on comfort tyres, still possible to win with crazy acceleration and corner speeds by downforce.
Pick the Miata ND15, engine swap.
What propably is "best", maybe Suzuki Cappucino with engine swap, or Audi Pikes Peak, but my experience of those is limited to Tokyo 600, so I dont know how they behave on different tracks with different setups.