This is a question for the older guys out there. If you were to compare the processing power of the ATmega32U4 to that of personal computers how far back in time would you have to go to go for it to match the best computers on the market?
How do you define 'best'? Do we count mainframes? To be the best computer in the world you'd have to go back to the 1950s-1960s.
If we ignore RAM size, the Arduino CPU is more powerful than a typical early-1980's home computer (Sinclair Spectrum, Commodore 64, Amstrad CPC, MSX, etc.) and probably more powerful than a 4.7MHz 8086 PC.
I'd say it loses the lead around the time of the 80286/Atari ST/Commodore Amiga, so ... 1985-ish.