Prestidigitation. It was my favorite spell to work with, as it was more or less a spell for flavor, compensating all the simple, yet clearly magical things wizards and such can do. I used it to entertain, to flirt, to make myself seem like a god amongst savages, and to try to make myself seem like a god amongst more modern savages, only to be tried as a witch and sentenced to death by incineration.
Most of the time, though, I just like coming up with interesting ways to cast/use/describe my spells. My DM was pretty lenient on how creative I could get, so my fireballs looked like a blue mushroom cloud blooming with dragon's wings, my Cloudkill came out of my mouth like a yellow fog, and my Magic Missile wand looked like a lotus blossom on a rod, the petals firing out as projectiles as I used them up.
So pretty much, any spell I can get creative with (most often in a way that doesn't really affect the game), I can have fun with, which makes Prestidigitation a Swiss army knife of character developing goodness.