I though about this a while. At first I was going to say there is not one best spell but att he end of thought of it. Symbiosis. You not only igve yourself an ability but you also give your friend one, how awesome is that? Sometimes it does not give us the best one but when it works it is great. It can be a complete game changer.
The other two I thought about were cyclone and monk teleport. Both are very good, but I am going to have to give it to Symbiosis here, it is so versatile.