The ones that will stand the test of time are the ones the look the coolest. It's that simple. There are a lot of different modeling amps of varying sound quality, but the ones that will be classics are the ones that look classic. Examples: Fender Mustang, Line 6 Flextone. The ones that look stupid, regardless of how they sound, will be looked at as toys in a few years. Examples: Line 6 Spider, Peavey Vyper.