Old Rhinebeck Aerodrome's Hispano-Suiza-powered Jenny. Until recently , the only one in the world still flying. Not the only flying Jenny, of course- the only "Hisso"-powered one.
Now there are none- they grounded it because it's "too rare to risk flying".

I've also seen the Canadian Lancaster fly... one of only two airworthy Lancs, I believe, and the only one you can board on the ramp for a tour. That was a treat.
Then there was Jimmy Franklin's Waco with the jet engine slung underneath... words cannot describe the sight- and sound- of that thing with the jet lighted... ! Sadly, both the Waco and Jimmy were lost in a midair with Bobby Younkin at an airshow in Moosejaw not long ago.
And another bittersweet memory...Jim Wright's excruciatingly perfect Hughes H-1 replica , which I saw him taxi at Oshkosh just before he departed on his last flight, during which he crashed on his way home to Oregon. He and that airplane were both one of a kind.
Speaking of An-124s, I see the Volga-Dnepr Antonov at KEWR from time to time, but still haven't caught it in the air... must only fly at night, or something...