I completed last year's in Scryer Prolog and it was a joy. Some problems were almost impossible due to the lack of mutation (Karger's algorithm comes to mind), but file parsing was a breeze and I find Prolog programs generally beautiful. My favourite syntactical feature is the full stop at the end of clauses.
Erlang lifted it as the comma, semicolon, dot convention. When I was writing a lot of Erlang I found myself wishing it was in other languages. After not writing Erlang for a long time, I wrote a few functions recently and it was jarring. Then again I usually prefer the conventions of whatever language I'm using most at the time, unless I really dislike the language (Javascript).
