The Blue Box Athearn SD40-2 in 1984 Olympics paint is, to me, an iconic HO locomotive of the 80s.
Detail is definitely dated but still decent, and the drive mechanism is simple to fix. I love that they are American made, in Compton, not far from where the 1984 Summer Olympics took place.
These are still cheap for what they are, but there are fewer and fewer mint originals available as time goes on, so I recommend getting one while they're still somewhat easy to find.