Hmm...in the past, spiny eel, banjo cats, tiger botia, common pleco. The pleco I have now never hides, I see him more than just about anybody. I also had a tank and dropped a tiny crayfish in there, saw him 3-4 months later and he'd grown to 3" or so.
I used to own a random pleco that I would see for about 3 seconds once every 2-3 months. I gave it to my roommate for one of his tanks, and haven't seen it since. I hated that fish.
Currently my clown pleco and one of my peacock eels are pretty good at hiding, in the past, another peacock eel, a bumble bee catfish and a brown ghost knife fish, I rarely ever saw them until I found their corpses Well I haven't found the BGK yet but I had all the decor out last month and raked the sand and didn't find it so I'm assuming it was eaten.
For me it's my Spotted Raphael and my M. uranoscopus cats.
I bought my raph when I started high school and I still have him today (about 13 years) and I am dead serious when I say that there have been times when I haven't seen him in over a year.
My uranoscopus cats are the same way, but atleast their new hide spot is somewhat in view so I can sort of tell that I have 2 of them.