Sudowoodo and Arcanine are not legendary, no matter what anyone says. The only legendaries in the game meet the following criteria: There are only one of them in the game and they cannot breed. There is only one Sudowoodo in the game, but you can breed it. Ditto and Nidorina/Nidoqueen cannot breed, but you can get more than one.
As for the worst Legendary - Articuno, Entei, Moltres, Raikou, Regice, Regirock, Registeel, Suicune, and Zapdos all tie for the worst total base stats at 580. Of that group, Entei and Registeel are at the bottom due to their limited movepool, and of the two Entei has the worst type.
The best Legendary goes to Groudon by a land slide, or better said, a QuakeSlide.