Of those three, the S2000 is the most sporting. Sure, it's not the fastest on the road, due to low torque. As a whole package, it offers the best experience.
The 2.2-litre revving past 8,000RPM doesn't hurt, either.
350Z - best all around of the bunch.
S2000 - great in the twisties but doesn't really have the power
RX8 - Looks like shit (subjective I know), no power/torque, Renesis should never have been made.
I can't stand the sound of that Honda when it really gets up there. It gives me a headache. The interior is really nice and it fits me good though. But the handling is more serious and less fun.
The Nissan looks like a dog and I've heard it doesn't handle that great (never driven one). Hate the interior as well. Sounds okay though.
If I had to choose one right now to live with forever I'd go with the Mazda. It sounds great, handles really nicely, looks the business and is more practical than the other two (barely). The maintenance costs are probably higher but that's the price you pay for having the one you want. And that engine is so smooth.