My favorite engine? That's like asking which one of my kids is my favorite child!!
For sheer pulling power, it's my Rivarossi 4-8-8-4 Big Boy:
Close behind is my Bachmann Spectrum 2-10-4:
It pulls very well, and is it will move at a crawl.
When I have the grey UP passenger cars on the rails, my Bachmann 4-8-4 Northern gets the call:
My favorite switcher is my IHC 0-8-0:
Union Pacific had exactly one (1) 0-8-0 switcher, and they had it, quite literally, by accident. Seems one of their 2-8-0 Consolidations got into a fender bender and had it's front pilot demolished. Rather than scrap an otherwise good engine, UP removed the pilot and turned it into a switcher. It was numbered 4500. Gonna have to renumber this engine, one of these days.
For industrial switching jobs, I turn to my Bachmann 0-6-0:
This engine came with a solid cast on plastic coupler that was totally non functional! Why Bachmann did that to a switch engine is beyond me. My local trainstore owner put a knuckle coupler on the front for me. He did a good job, but it angles up slightly. I'm going to put a shim between the frame and the footboard assembly to bring it down a bit. Works fine as it is, though.
So, depending on the task at hand, I have a lot of favorite engines. Just like I have two favorite kids.
Hopefully, the budget will allow me to build a layout to run these on soon. Putting down EZ-track and taking it up again gets old after awhile!