Which cryptocurrency has the smallest block time? What is the lowest block time of any cryptocurrency? I don't mean the time of an individual block, but the target time per block.

What is the shortest reasonable block interval?
There was a coin called Flashcoin that tried to do 6-second block times. I don't think that's being actively mined anymore as another Flashcoin appeared a year later (60-second block times).

Ethereum is planning a 12-second block time. Vitalik makes an argument for why that's probably the minimum for now here. Of course, ethereum hasn't been released yet, but Fastcoin with its 12-second block time is probably considered the lowest block generation time currently.