Choosing from my roster, they would be:
#5: ALCO S-2 Marked with the Western Maryland yellow stripes. Great little switcher, even if the smoke just wafts out instead of being fan driven.
#4: SD35/GP38-2 The former in WM red, white & black, the latter in Md. Midland blue and orange. They look great and sound great.
#3: AC4400CW Just a brute of a machine. In CSX livery. Pulls anything I put behind it. Love to hear it start up.
#2: F3/F7 What can I say? Everybody should have a set. B & O blue & yellow. It's just a railroad icon that represents the age of the diesel.
#1: FM TRAINMASTER Even though recently purchased, it's not my number one only because it's new to my railroad. It's black, PRR marked, and is massive, even in O scale. Have never seen one in person, but I would think if you saw that coming toward you and felt it going past you, it would take your breath away. What an impressive machine. MTH did a great job on the sound file for it, too.
Mike