Three-way tie between North American P-51D (or H), Focke-Wulf Fw-190 (A series with radial engine) and Me-262 (but Me-262 is cheating anyhow even though the OP allowed multi-engine jets

).
Just plain gorgeous all three of them, I get tears in my eyes when I see propper era footage showing these planes in flight. Each is a representative of their own type of engine. I'm not sure I could pick just one but I used to be very partial to the Mustang ever since first seeing one in a book.
Other planes I also like alot but wouldn't put in 1st place:
- Hawker Tempest (3000+ HP when operating under emergency power ? 'nuff said)
- Supermarine Spitfire (if it wasn't for the asymmetrical radiator in early versions ...)
- Fiesler Fi-156 Storch (piloted by Hanna Reitsch, used in Gran Sasso raid)
- Heinkel He-162
I dislike the appearance of most Japanese planes (few Army planes with inline engines being rare exception) and most US planes, they were all just downright fugly, just like their Australian counterparts. Soviet planes were utalitarian, nothing enchanting about them. Some Italian planes did look very nice but I'm not familiar with them well enough to include them in my vote.