I have found the Ferrari 458 to be pretty much the best handling supercar I've come across. As standard, it appears to be the best balanced, in comparison the Mclaren MP4-12, and particularly the Gallardo, feel like they have a more rear-biased weight distribution, which can make them more of a handful on corner entry. I find the SLS quite good to drive, if a little more understeery than the 458. The Nissan GTR, in my opinion, is the easiest supercar to drive quickly, as you can drive it far more aggressively without wrapping it round a tree.
Admittedly, I haven't driven an Enzo yet, but I get the impression that many people who complain how it handles don't realise how fast it moves in a straight line, then are surprised when they can't make it through the next corner. Even the Veyron (which I have driven) has quite a nice balance to it as long as you take in to account how quickly you are approaching the corners.