Super Mario Galaxy 2
Really, there's nothing else worth talking about after this, because that game was just so horrible compared to the rest of the series that anything else would either be "not bad enough" or the rest of the series would be "not good enough". Well, I've heard that Sonic 2006 could compete, but I've never actually played it and I'm not LMS so I'm not going to speak just from seeing a Let's Play (also, I never got terribly far in that Let's Play anyway.)
I guess I could mention A Link to the Past, but that wasn't actually that bad and the Legend of Zelda series was never really a huge draw for me anyway. Worst Sonic game that I actually have played? Eh...probably the original, because no Spin Dash is just plain stupid.
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"Everyone has a dick although it may not be physical. You just have to learn to access it within your mind." - blindhobo13