1200 gallon. Saltwater tank. Owned by a guy we did a stainless steel sheetmetal job for in Pebble Beach. Awesome guy. Impressive tank to say the least. He has a huge indoor koi pond as well.
My all time favorite tank doesn't count for this but it's the 120,000 gallon tank at the Seattle aquarium. It has a 20' x 40' viewing window that is about 12 inches thick. Simply amazing.
Eh, I've not been to the homes of many fishkeepers. The biggest tank that I've ever seen was a 90,000g shark tank at the Coney Island Aquarium, but I didn't vote since it's not a personal tank.