Not having driven any of these (and I might never get a chance either, but I'm not complaining) I'd personally go for the M3, in saloon form. The ISF looks too fat and the Merc looks flobbery and chintzy. I know it's probably the best compromise here, but still. Truth be told though, my pick of the range would be a 135i coupe. But that's like coming to a gunfight with a knife :$. sami slinks away...
p.s. A Vishnu tuning kit will put the 135 ahead of even the Benz though...