The utility of a REPL isn't predicated on the expressiveness of the language you use. I would argue lots of languages which have far better REPL experience than Python, are also far more expressive than Python.

Expressiveness and complexity is not quite the same. Python has become fairly complex but without actually becoming a lot more expressive. E.g. Python does not have anything close to the meta programming abilities of Julia, Clojure, Scheme etc which are better REPL languages.

The fundamental reason the REPL in Python is bad is that Python is an object-oriented language and uses significant white space. Both those factors contribute to making the Python experience less than stellar.

No, there is no best language but the appeal of a language is often about ecosystem and community and this is where Python wins today. I don't think the language in isolation stacks that well against more modern dynamic languages like Julia, Clojure, Elexir etc.