I had to rise from the depths of lurking to post in this topic.
Most themepark trains don't interest me that much and usually only get ridden once everything else has been done, but there are exceptions to these, especially when steam is involved! Dollywood easily wins in this category with its impressie locomotives. I don't care all that much for the history of railways, but the sights, sounds and smell as it works its way up the hill is impressive for a themepark.
Outside of parks, my favourite goes to Moors Valley Railway by Bournemouth, UK. I am biased as I used to work full time there as a steam engine driver during the summer and still volunteer my time a few weekends a year to drive as its so much fun - I've even had the pleasure of driving visiting CF members in the past!
A brilliant little family run country park railway where all the staff and volunteers clearly love the work that they do. They have about 20 steam engines, most of which are all built by hand by the people on site and its one of the only fully signaled narrowgauge railways in the UK, with (very!) steep hills, tunnels, crossings and a complicated layout for a railway of its size.
Since moving away from the south i've taken up volunteering at Nene Valley Railway (Cambridgeshire) on the footplate driving & firing. Its quite unique as it supports a wider gauge and has lots of European rolling stock so is used in film & tv; although it doesn't really have the same character or charm as other railways out there - especially as you start to approach Peterborough city centre.
Honorable mention to the Welsh Highland Railway and Ffestiniog in Wales. An incredibly impressive railway now to the two are linked together with stunning views and some impressive locomotives (The Garretts on the WHR are still my favorite locos!) Other mentions go to Swanage Railway, Mid Hants (Watercress line) and the Severn Valley Railway.
