Anything in the Swiss or French Alps.
I took the Lyon > Chamonix train about 3yrs ago and I'm still drooling when I think about the views.

Honourable mention goes out to Machu Picchu express that starts in Aguascalientes, but I don't recall where it ends. I'm sure it's nice during the day though, but we left at 4pm so saw very little and then had to endure the "fashion show"...

I don't recommend though the HK > Beijing train. little in the way of scenery and with only 2 channels, unless you speak Chinese you'll get bored...