What is your favorite aspect of model railroading?

[nachoman]

daydreaming Seriously, I could kill hours just thinking, browsing catalogs, or looking at railroad photos on the net.

If you are asking something more specific to model railroading - I'd have to say weathering or adding groundcover. I find both of those two tasks relaxing, and the results make a HUGE difference in appearance. If you spend an hour adding ground cover to a section of your layout, you go from having a plywood mesa to something you could take photos of in a short amount of time.