SR-71 is beautiful in the sense of "form follows function". To understand the beauty you need to realize its capabilities. And yowzers what an amazing airplane.
Columbine is just beautiful. Even to folks who know nothing about airplanes.
P.S. I have no one favorite. It's like cars... sometimes a T6, other days an SR-71. And other days a DC-3. And then I defer to a Ryan ST-A...
https://sss.airliners.net/photo/Ryan-STA/1991929/L/. Or a Piper Cub. (TR, Alfa GTV, step-down Hudson, W108 Mercedes, 911, ...)