As per the title, what's the purchase you made that made you most regret your decision? It could be anything from a console, to a game, a peripheral, DLC.
Mine has to be the PS Vita, which hurts because as a console it shows so much promise. I'm not a big JRPG fan, and really the selection of great games on the handheld outside of that genre just isn't big enough. I've had some great moments with it, playing Metal Gear Solid HD again & Gravity Rush.. but there just isn't enough there to make me not regret my purchase.
I do wish Sony supported it more, and that there were more games designed specifically for the Vita rather than lazily ported over, because it had the potential to be one of the best handheld consoles ever.
So, what's yours?