Best in terms of power?Armor of Reality (an Armored Cloak that stacked with Armored Cloak) was pretty sweet.In terms of fuzziness, there were some awesome regionals at Arisia this year. One allowed me to count as two people when sailing a ship and another allowed me to immediately board the nearest ship on the event site no matter how far away it was or what terrain separated us. Good times.
As a regionalist for many years, I came across many powerful spells, but they usually aren't the really FUN spells. My three all-time favorite regional spells have been:1) Phantom Shovel: verbal "Actually, i do have a shovel" effect: you have a shovel2)The spell that I had in the dreaming that let me fly by flapping my arms3)Mystic sword: allows one to summon a sword that one can use and doesn't count at all towards weapon restriction. When you cast it you got a balloon sword.
Well, I am a fan of Shane regionals in general. One of my favorite effects of a regional, however, came from one of the Rhiassa Questing events. I believe 1st or 2nd regional was Rock Skin, which was effectively a Protection from Boulder. At one point, three of us with Rock Skin (one of us might have just had Pro. Boulder)were on one side of a cave wall. A boulder was thrown over the wall at the crowd and the three of us simultaneously screamed "Protection!" and slammed it down into the ground. It was amazing.