I'll admit to being a terrible interviewee for this one, as the favourite aircraft (incl. glider, etc.) tends to be the one that gets me in the air on any given day. Like PC, I think it's very mission dependent and I'd also add that, in a life of flying, it can be rather epoch dependent, too. Experiences that come as revelations at one time of life can be pretty hum-drum later on.
But I'm going to take the bait anyway and declare a few favourites. The humble DR400, flown off grass runways in France 25 years ago, was a revelation for an Aussie trained on Wichita spam cans. It seemed at the time to violate the opposing principles of agility and stability.
Later on, a much-loved PA24-260 was just about the bee's knees for touring Australia. Never a big fan of Pipers, but this one made me think much better of them. Then there was the slick little Decathlon that convinced me for a little while that I knew about tail-wheel aircraft, until the next flight in the Auster or DH82 at least. And hard to beat the trusty little C150 on a power-off approach into a bush strip on a frosty morning, the grey kangaroos looking puzzled at the intrusion. Or a slick C172 for a landing on a deserted beach in Southern Tasmania, or a a flight over the volcano in Hawaii. For gentleman's aerobatics the Airtourer 160, or an RV6 come to mind.
These days? Depends on the mood but am still smiling after the morning bimble in the P2002JF. An hour's fun at 100 kt on 3/4 jerry can of premium mogas. As an old pilot friend used to say "you wouldn't be dead for quids, boy!"