Any of the with enough $ in them!
Porsche, obvious choice.
Pros:
weight distribution
factory suspension design
great chassis
Cons:
EXPENSIVE (yes I'm yelling)
So-so aftermarket (front engine rear drive)
engine design, oiling issues and very pricey for parts
240z
Pros:
small and light
good factory suspension
good chassis
Cons:
power options
aftermarket
most of those models have cancer that could have structurally damaged it
SN
Pros:
very strong aftermarket
power goals easily achieved
cheap everything comparitively
Cons:
not good out of the box, suspension or power
needs quite a bit, specially for competition
chassis is about as stiff as wet spaghetti
Depending on what you really want to do, think ahead. What is your goal ... if it's just the occasional track day. Use your current SN, why not? If it's to have a dedicated track car that you'll tow to the track, beat the snot out of it maybe in a TT class or auto-x, any of the options above will be fine. Just how much work and $ you have to go play. Now if your goal is to wheel to wheel race one day, you should check out a NASA or SCCA event and see which classes are popular. Because if you are the only running in your class, wheel to wheel racing is about as fun as eating vegetables.
Other EXTREMELY popular and inexpensive cars on the road course are 3 series BMWs, any vintage. They do have a great aftermarket and can fit into many race classes depending on their prep level.
Hope this helps!!