This is XBox only, my brain can't think back to some of the utter shit I bought for the Atari 5200 and NES.
Raw 2, although State of Emergency is a close second.
What's REALLY sad is, I should have KNOWN SoE would suck - I bought the XBox one, which came out well after the PS2 game. I hadn't actually played it, just saw the "slightly above average" reviews of the PS2 game and figured it would be ok since it was 20 bucks. I thought custom soundtrack support would make the game better - nope. Sold it and got a good chunk of it back thankfully.
Raw 2 is my biggest screw-up. Went out on release day and bought it, despite the voice in my head that said "those preview videos you saw look an awful lot like a more polished version of the crappy the first one..." I think I paid 50 bucks for it. Sold it a week later and took about a 17 dollar hit on it. Oh well.
For those that think Halo 2 is the biggest waste of their game-buying careers...yikes. You either buy nothing but completely amazing games for rock-bottom prices, or you're exaggerating a bit so you can take another dig at Halo 2. To each their own, I suppose.