2001: N/A since I wasn't a fan (for all N/As)
2002: N/A
2003: Takanuva (confusing bike, recolored Gali Nuva clone)
2004: Vahki.
Rehashes of the Rahkshi! (that would make a good comic name

)
2005: Hordika, but the Visorak's excellency corrected this
2006: Umbra (you know what I mean)
2007: Hewkii Mahri. Just look at his no-flow, bad colours and random spikes.
2008: Vultraz
2009: Can't really say, this year's sets are excellent
2010: Takanuva. No Gold and two irrevelant weapons, period yet his 08 set was so great. Guess that balances it.
I really do not agree with 2003 worst set. I thought Takanuva was decent...
2004 - I have a Vahki, and he wasn't that bad... I really do not think there's any awful set...
2005 - The Hordika were lackluster. The Visorak had a decent color scheme IMO.
2006 - The Piraka had really weird arms and joined spine/neck, but that's it.
2007 - Agreed. I have Hewkii Mahri, and he was horrible. Awful color scheme, weird arms (no hands

), and overused torso. At least his pieces are great for MOCs...
2008 - Has got to be Tahu Mistika. Reused torso, and very few new pieces. Also Kopaka Phantoka.
2009 - No horrid things I guess, only I do not like how the randomly put silver on Malum's claws...
Or maybe Toa Mata Nui.