Personally I feel that Alex Ferguson is the best manager, with pretty much every club he has won the league with, look what he did at Aberdeen. You can't achieve so much and not be the best manager.
Wenger is good, he spots potential but not all of his buys are top class. He's bought some woeful defenders and some awful goalkeepers (can't stand lemon hes the dodgiest one of the lot).
Now I'm not saying Fergy hasn't had his share of bad buys but I always look at it from a British point of view, whose done more for British football, and its blantantly Alex Ferguson.