Okay, so here’s the deal: in an ideal world, we’d want most fun rather than funnest (there is a good discussion of this at World Wide Words). And yet, because it’s advertising, I’m inclined to consider it through a slightly different lens. Funnest is lazy, light-hearted, casual. It evokes a certain feeling: we’re having so much fun when we use our iPods, we don’t really care about grammar (I know, it’s hard to imagine).
So do I love it? No. Nor do I hate it. For a second opinion, check out Grammar Girl. What about you: does this kind of intentional misuse cause you to grit your teeth, or can you grin and bear it?