I guess the 'busiest' part has been perfectly answered
About the 'best' part, this has really been subject of quite a number of threads, but whatever.
CDG: I havn't been there too often, but everytime I got there I was heavily disappointed by both airport and staff.
MUC: I like it really (maybe because I live there) and I think RoyalDutch has some good points, but there is room for improvement. I wouldn't call it THE best, but one of the best airports I have been to.
I also think that AMS is a really good airport, same goes for YVR.