I've always found Roy G. Biv to be the dumbest mnemonic. It's actually easier to use the colors as a mnemonic for remembering the name than the other way around.

Music theory mnemonics are a close second. There are so many of them, many of them sharing the same words in a different order, that it's as hard to remember which mnemonic to use when as it is to just remember what they stand for. Especially the Dutch ones all seem to revolve about feeding monkeys.