Figured I'd make this my first post.
I'm a Wankel lover, personally. My tuned ~550hp RX-8 is an utter joy to drive - the tail never slides out despite the fact that I tweaked it for massive amounts of oversteer. But my favorite car of all time has to be the 787B. There is nothing more fun than taking that monster of a car around the Circuit de la Sarthe.
Of course, there's something to be said for my first car ever, my old metallic orange MR2 from GT4, recreated in GT5. Fun little thing. Also a hell of a car to learn the ropes in.