Well, Star Wars is the only thing I know.
For the best, I would go with the AT-TE. Much like the more recent Trade Federation MTT (the one that was released in 06? 07?), the Clone Wars AT-TE is a great improvement over the old one in the 4000s series. It's just a nice display piece, detailed, and comes with Rotta the Hutlet! Maybe it's just my obsession with Star Wars walkers though. :)
For the worst, I would be tempted to say the Twilight. But, Im going to give LEGO a free pass. I thought the model is a very good representation of what's in the movie, but what's in the movie is terrible looking.
So I'll say the worst is the Magnaguard Fighter. Did we ever see it in the movie or cartoon? Overall, it just feels bland and boring. The minifigs are cool, but '09's Count Dooku Solar Sailer now gives you two Magnaguards.