I would say albino bristlenoses. They stay smallish (4") or so, and are good little algae eaters. They will rasp the older leaves of Echinodorus (sword plants), especially if they have some tasty algae on them. As a previous poster mentioned, they keep the glass very clean, so I put up with having to trim off the odd sword plant leaf. They do not become aggressive as they age, either, or go after fish to feed on their slime coat.
They are also very long lived. I have one in my 66 gallon tank that is at least 14 years old.