I've taken Zurich - Budapest, and Paris - Vienna sleeper trains years ago, was a great experience. (Though the Zurich - Vienna ride is so beautiful you'll want to take a day-time train, which I have on other occasions.)
Not shown here, but I've also taken sleeper trains across Russia (Moscow - Novosibirsk) which was quite a fun experience. Get to meet all kinds of people, drink lots of tea / vodka :). Always wanted to the 7-day Moscow-Vladivostok but didn't quite make it. This was early 90s though -- no idea what the experience is like these days.
Not shown here, but I've also taken sleeper trains across Russia (Moscow - Novosibirsk) which was quite a fun experience. Get to meet all kinds of people, drink lots of tea / vodka :). Always wanted to the 7-day Moscow-Vladivostok but didn't quite make it. This was early 90s though -- no idea what the experience is like these days.