What people consider the worst depends on personal preference. Sunkern has the worst stats in the game, while the VF bugs (Caterpie, Metapod, Weedle, Kakuna) and Magikarp have the worst movesets in the game. It depends on what is more important to a person; stats, or the moves the Pokemon learns.
In my opinion, one of the worst Pokemon not to be mentioned yet is Magcargo. I mean, seriously. This has to be the most pathetic excuse for an evolution ever created. You have to go through the pain of training the equally horrible Slugma all the way to Lv. 38, and this is your reward for doing so? A Pokemon with virtually no Speed at all, a horrible Attack stat, making most of the moves it learns like Rock Slide totally useless, a mediocre moveset, with Flamethrower being its only benefit, and a ton of weaknessess due to the absolutley ridiculous type combination of Fire/Rock. The only thing it has going for it is it has a high Defense stat, but what good does that do, considering it has more weaknessess than almost any other Pokemon in the game?
I honestly don't know why Game Freak even bothered with this, considering they gave Magcargo no benefits to using it at all. I guess it was created for the people who have beaten the game a couple of times that are bored of playing, so they give themselves a challange by using such a bad Pokemon.