Some of my favourite classic-era open sight guns (most of which I have owned in some form or other):
- Airsporter Mk1.
- Webley MkIII.
- Diana 50.
- Walther LG52 (rare, never had one, would really like one).
- 1950s/early 60s Diana 35 and 27.
It is a sin to scope any of the above (if they have scope rails/grooves). Except the D50, as they were current past 1980. They were all designed and stocked for irons.
There is a kind of irony that many of the 50s sights are steel and in some cases works of the machinist's art, but the later 70s alloy and plastic ones often give a better sight picture, especially on paper targets. They are better as sights, just less charming to use. That is certainly true of the Dianas.
If you mean "iron sights", not just "open sights", the diopters on 1950s German rifles are lovely. And the PH apertures on things like MkIIIs are pretty good too. And a Webley Tracker with a front sight (not barrel weight) and a Williams peep rear is a fun thing, as is a Sheridan with the Williams.
Do also consider the HW55. I am pretty sure there was an open-sighted base model. That could be interesting.