I have to break this up into three parts to be fair
My number one favorite fish is Characidium fasciatum (banded characidum, darter tetra). Not a catfish, but it is a cool little bottom-feeder.
My favorite catfish are Tatia perugiae. Not your typical catfish - they will scavenge the bottom some, but are primarily surface feeders.
My favorite algae-eating fish are Otocinclus cocama (zebra otos).
If I could only keep one of the three........well, I don't think I could choose!