In a list that you'd probably never think you would see and I haven't been able to find, Kevin Gold, an agent who represents special-teams players and who knows personally every snapper in the league, and runs Longsnap.com, has said that the Bears 10 year vet out of Duke is the best long snapper in the game.
As is mentioned in this article is the most famous, if famous can even be used here, long snapper in the league.
He started a web site called LongSnapper.Com awhile back to help kids who have interest in the position. It gives tips, advice and drills on how to improve.
I am the last person in the world who can accurately judge a long snapper, but it is nice to know we have the best.
On a more personal note, I have interviewed Mannelly and he is as nice a guy you could have ever hoped to meet.