Very diffecult question. First is which part of wales you are staying in and how you are travelling there. The majority of the NG lines are in mid to North Wales which is not so easy to move around (due to mountains and roads) as the southern part, but is very accessable for Manchester airport more easily than Heathrow (London).
The best railway for a combanation of vgood sceanery and locomotive veriety has to be the Ffestiniog. They run daily about once an hour and climes from next to a large (UK standards) eastury to slate minning country at a constant grade of 1:90 - 1:60 (about 1.2 - 2%) You will see a minimum of two steam locomotives on normal trains all year. The line also includes a spectacular spiral.
However Other lines are better at sceanery - but have no veriety of locomotives and low time table, or have a good veriety of locomotive to be scean but not so good sceanery.
However (again) if you are jurneying from the midlands you will pass several other lines (or near to them)on your route to the FR. The most notable being the 8mile Welshpool And Llanfair Railway. We are visable from the main Shrewbury - Dougeddlou (wronge spelling sorry) road are offer a two hour return ride with this your three working locomotives, one original, one from Africa and one from Finland! other locomotives are normally also visable and you will find it easyer to move round to your leasure. Ask any of us on duty on your visit and we will arrange gouded tours though places off limites on your own. We also have two miles of very steep (1:29/4%) grade which is the steepest in the country. Most visitors to wales call and ride on us as a break in there journey further in. You may find because of this you end up visiting the FR as well.
Don't hesetate to contact me for any further info
Owen Chapman (W&L volluteer)
Welshpool and Llanfair Light railway: WWW.wllr.org.uk
Ffestiniog Railway: www.festrail.co.uk