Off the top of my head for commercial operation:

There are a ton of biplanes from the 1920s you can buy rides in. A friend of mine has a 1918 Curtiss Jenny that's mostly original. It's still flown and displayed at shows.

The airplane with the most time on it in the world is a DC-3 with over 100k hours. It's had the engines replaced with turboprops, like many of them.

When I was a student pilot in the early 90's a B-17 offering tours flew into my airport. Unfortunately I had my first solo cross country that day and no way was I going to miss it!

https://en.wikipedia.org/wiki/List_of_surviving_Boeing_B-17_...

there are prol a few C-47s built during WWII still carrying passengers in 3d-world countries.

all the B-52s still in service are B-52Hs and deliveries of the H didn't start till 1961.

I'd go for the DC3.