What do you consider looking good? What temps will your water reach at night in the winter?
The biggest pleco is the LDA65 Titanic pleco. Oddball use to have one, and they max out at near 3'. IMO the biggest good looking pleco would be megalancistrus species. Of the species that look really good, the biggest would be hypostomus luteus in its gold stage, or royals (particularly L027, and L330).