Really hard for me that one, we've loved all the tours we've done although they've all been at Disney so havent tried the other parks yet.
Keys to the Kingdom was such great value and we did SO much in the time including going backstage.
The all day Backstage magic was so good we've done that several times :)
then there's the youltide fantasty one which was very christmassy and great in its own way.
There's a fair amount of duplication between them all though. I guess if I REALLY had to choose just one it would be Backstage Magic