i know what your trying to get, you want a car that in stock form is a weapon... the best 'bang for your buck' sort of thing. But a cars power is easily changed with a bit of boost here, and an intercooler there.
At the same time dont discount other factors like how well it puts the power down. Also the quality of the car itself, both interior and engine wise aswell. I said go the S15 because it really is a good looking car in dead stock form, and the amount of mods on the market for it are mind boggling. Its a light car, 6 speed box and huge engine potential.
but remember you are on a skyline forum so i think most of the advice your going to get will be biased towards certain cars