I use a simple bow like you would tie your shoe lace with.
The only thing to remember is to pass the rope, tape etc around the tree and tie the bow just below where you passed it around the tree.
I have used this knot loads of times because it's quick, easy to tie and has never let me down. As has already been said it is also easy to untie and if you are worried that one of your mates will ubdo it for a laugh, just tie a double knot like you would with your shoes... hey presto. Job done..
Why make life difficult with complicated knots when a simple bow will do the job.
Cheers
Phill