Of the ones I can't have I'd say the Curta is my favorite...It's just so cool.
If you do find one, remember that it will most likely take many hours to rebuild, with expert hands, in case there is any mechanical issue.
The Curta is super cool because it is so small, but there were a wide variety of mechanical (or electro-mechanical) calculators over the decades. Friden and Brunsviga had some cool models.
I've wanted an HP-15C, because I've never had an RPN...but I can't justify the high used prices.
There are a couple of companies making replica 15Cs, including a full-size and a compact.
https://www.swissmicros.com/product/dm15My favorite is the one I used most in school, the HP 48GX. I've been aware of follow-on models that are supposedly faster, but each time I held one in my hands I was disappointed in the feel. Not just RPN entry, but RPL programming is really nice on the 48.