Ooh this is tough. If you had asked me a couple weeks ago, I would have said the Devi Ever Improbability Drive, both the crazy amounts of sounds and the corksniffer/unobtanium factor. But man, the Philosopher's Tone really blew me away last night at practice. My tone sounded so much fuller and punchier, it gave me sustain for days, and my highs were really singing too. So much so that my drummer even said something to me afterwards about how good it sounded. It might not be the sexiest purchase, but it's just so damn useful and sounds great.