Best new sports car for under 30k?

Looking to buy (potentially) a: Camaro, FRS, BRZ, Mustang, or someting else?
Any Suggestions? Why?

It really depends on what sort of driver you are.
your options are

Mazda MX-5 Miata, Scion FR-S/Subaru BRZ, Hyundai Genesis Coupe, Ford Mustang, Chevrolet Camaro, Dodge Challenger.
you can also find other sporty cars like the Honda Civic SI, Hyundai Veloster Turbo, and many others. These will offer front wheel drive but will still have improved power levels, a sporty suspension and can be lots of fun. For this though, I'll focus on rear wheel drive cars because you seem to as well.

when you look at the range I have listed above, you will notice that there's a large distinction between the japanese and american cars. The japanese are down on power but are also down on weight. They focus more on handling rather than straight line speed. In curvy roads or a racetrack, these cars will feel great. They will be extremely easy to control even past loss of traction. On the road where lines are much straighter, they will lack acceleration but they will feel great when you push them in the corners.
in fact the Miata is so good for handling that there are many Miata only racing leagues.

the American cars on the other hand are more about highway performance. The cars are much heavier and larger. Their suspensions a lot higher and softer which means more body roll in corners. When you compare something like a miata with 167 hp, with a base model mustang or camaro with over 300 hp, you will see the huge difference in priorities for the american cars. On a straight road, the american cars will be much nicer. The suspension will be more comfortable and you will have more storage space and seats in the americans. Push them in the corners and you'll quickly find that their weight is a huge disadvantage but on the roads they will be pretty nice.

now my preference actually goes to the odd one out. The Hyundai Genesis Coupe.

the genesis coupe is the midpoint in all aspects with a couple benefits not shared by the others.

the genesis is larger and heavier than the japanese but it is smaller and lighter than the americans.
it is a good mix of the two offering better than normal power and handling, not power or handling like the others.
best of all however is how easy it is to make a hyundai better.
the genesis coupe comes with some factory available upgrades that make it much cooler.
you can get lower and stiffer springs as well as anti roll bars and engine bay chassis strut bars installed by the manufacturer. You can also get an option with a limited slip differential, brembo brakes and a sports exhaust along with the previously mentioned upgrades.
then there's the engine. The 2.0L turbo engine is by far the easiest to modify for more power. So much so that Hyundai engineers actually expected this and made the engine run a bit rich on fuel such that when users add in an improved intake and exhaust, they will still be running an optimal fuel mixture.
add in some ECU tuning and improved fuel injectors and the genesis coupe can put out more power than the american cars.

these handling and power upgrades can make the hyundai better for both directions of performance. In my opinion, this is very cool because you can have a balanced car that won't be as lacking in one area as much as the japanese or american cars. This balance of performance is in my opinion very cool.
because of that, my recommendation goes to the Hyundai Genesis Coupe.

keep in mind that you may feel different to the cars, do some test drives to see which car fits your style better and which you like more. Used Porsche 911. They may cost a lot of $$ to run but they're reliable, fun to drive, and it's charismatic.
Both the Camaro and Mustang are not true sports cars. Not a huge fan of the FRS personally Sti Evo si Honda rx7 Fd supra nsx

Used Porsche 911. They may cost a lot of $$ to run but they're reliable, fun to drive, and it's charismatic.
Both the Camaro and Mustang are not true sports cars. Not a huge fan of the FRS personally

Sti Evo si Honda rx7 Fd supra nsx