Seriously, if you've seen the use of a spell in real life, that's something for a whole 'nother topic.
The best use - or worst, if you were the DM running the adventure, was by a 14th level druid in an AD&D adventure run at some con at Columbia University back in 85 or 86.
The adventure was some sort of death trap. I don't remember much of it, except that at the moment in question, the party was taking a beating with more harm incoming.
The player of the druid: "This is a tightly packed dungeon. What ate the walls and floors and stuff made out of?"
DM: "Er, stone I guess"
Druid: "I cast Transmute Rock to Mud"
DM: "K, so what are you taking out? A wall?"
Druid: "Uhm, its a 2" cube per level, which gives me 14 x 2" cubes - 2" is 20' in real life, so that's 14 20x20 cubes of rock to mud - I'll take the room out in front of us, to the sides and behind that room. Oh, and i center it mid height in the room, to me sure to take out the ceiling and floor. Next round I'll dispel magic on the mud."
DM: "Fucckk!" Throws notes and maps onto the table. "Well, we're done now."
Yes, I was that druid ;)
Thanks to Sa Roe for the topic :)
So, what the best, most creative, disruptive but BTB use of a spell you've seen in your years of gaming?