If we're talking about a display tank I really like around the 100-150 gallon size for a large room like the living room. It's big and draws the eye but doesn't take up a huge amount of space. Okay, it takes up way more space than a 10 gallon does but for a large sized display tank it fits the bill.
If we're talking about convenience, I like using 10 gallons for quarantine, hospital tanks and breeding tanks. They're a very handy size for most things. I have several empty 10 gals I use whenever the need arises.