I've raced for 7 years against the best of the best (in my opinion) in the SEDiv.
If I were selecting an ITS car now, I'd have two absolute prerequisites:
1. EFI
and
2. Four wheel disc brakes.
As people have listed above, there are a lot of chassis in ITS that can win, and as Kirk notes, given the right amount of driver talent, development time, and dollars, I'm not sure there are many cars in the ITCS that cannot compete.
If you want a proven, tried and true formula, then the 99 ITS Miata, the E30 BMW (the E36 still has the restrictor to develop around), or the RX7 are probably the ticket. The Z cars (up to the 280z) are a known quantity, fast and still competitive, but those carbs and drums...hmmmmm.....
PM if you'd like. Need to know more about what you want to do. For example, RX7s are fast, how to develop them is well known, but they have a lot of maintenance requirements and can be finicky. E30s are solid cars with no real weaknesses, a good choice if you just want to drive. Same with teh ITS Miata -- blindingly fast at some tracks but at a disadvantage at others. Porsche 944S? Perhaps unbeatable power but weak links in the motor.
And so on. If you want to develop a car, I have some ideas. Note that none of them are "TR8"....lol....If you want to just jump in and race, I have some other ideas.