My favorite- right now- is my 1985 Trek 720. I think the primary reason it's my favorite is because it's a smooth riding bike with a 10 speed rear end. At this time, it's my only 10 speed.
My previous favorite was my 1985 Trek 620. I think the reasons for it being my favorite were (initially) the cachet of an 80s Trek touring frame, then the smoothness and the "give" that frame has. Later on, I built it with 6 speed Accushift and Command Shifters- that bike was/is really a dream to ride- Right now it's in the middle of getting upgraded to 10 speed, very much the same as my 720. After this, I'm not sure which one will be my favorite...
Trek 720:
IMG_0616 by
Dave The Golden Boy, on Flickr
Trek 620:
1985 Trek 620 by
Dave The Golden Boy, on Flickr