I have a Xbox 360, a quad bike and an iPod (although not a touch).
Xbox 360 - You say you have a Wii, DS, PSP and a PS2 and you don't play them. Why waste more money on a console that you won't play? The Xbox 360 is a great console, however. Maybe if you get the games you want, you may play it.
iPod touch - Yeah, there fun and all. But its an Apple product. Meaning in eleven or so months, it'll be obsolete again. Remember, Apple don't routinely update the operating system for older iPods. iPod touchs are cool with the apps, but you have to consider how quickly the novelty will wear off.
Quad bike - I hate physical activity...but I love going out on the quad. Just going down to the local field that council have allowed quad bikers to ride on, pulling wheels and riding over mounds. Its great fun, and its always fun. Its also beneficial to you. I'm almost 16 myself and am going to get my learner's next month, and I've already learnt a bit about gears, listening to the engine, just from using the quad bike etc.
So, if it were me, it'd be between quad and 360. For you, I'd recommend the quad.