I don't have any obscure favourites that I actually use. But I have a soft spot for a bunch of old, obsolete ones.<p>Niklaus Wirth's post-Pascal languages, including Modula-2 (and its offshoot Modula-3) and Oberon were fantastic. Their syntaxes fit on a single piece of paper. Modula-3 is an outlier, since it was developed without much involvement from Wirth, but it adds some nice stuff, such as generics. Oberon had a strong influence on Go.<p>As a kind of "lost language", Barbara Liskov's CLU (1974-75) is fascinating. Its syntax is Algol/Simula-like, but it comes with a lot of advanced features (iterators, generics, type parameters with constraints, exceptions, variant types, parallel assignments, inheritance-less classes, "everything is an object", garbage collection, no global variables) that eventually ended up in other languages, much later. CLU was never intended to be used, but as a testbed for ideas, and it never really was.