I agree with everyone who has nominated the Baron, the Queen air and the Aero Commander 500. The Chipmunk too, though I'm heavily biased towards Chipmunks as I learned to fly on them and hey, you never forget your first love!
I'd also offer up the Cap 10 and the Stampe.
But, very little beats the delights of a simple J3 Cub, on a calm summer evening, low and slow, with the door open and preferably on floats. Sigh.