Good to see this discussion still going, despite it being not what the original author intended! For me there are 3 locos that stand out as being all time favourites.
Being born in the 80's I never saw mainline steam and I make not secret of the fact that I prefer D&E. However, my favourite route (London Paddington to Bristol Temple Meads) just happens to go via Didcot Parkway at the wonderful railway centre. I have therefore been a GWR man ever since I first discovered the Castle class (until then as a lad I was LNER, well with A3's and A4's, you would wouldn't you?). The Castle therefore comes in at number 3.
Number 2 is the ubiquitous 08. I simply love the fact that such an old design could become so widespread and even now be so useful on the rail network. These unpretentious little locos only do 15mph, simple because that's all they need to do.
At the other end of the scale is my all time number one, the HST power car. As a young lad these always meant a long and exciting train journey and good times. The noise these things produce is also pretty special, almost jet like as they spin-up on accelerating away from the platform. If I ever one the lottery I would try and buy one, as the upcoming replacements on the rail network (silly multiple unit things, no proper loco at all!) simply don't have the soul. Preservation is a must!