I think that's a matter of opinion; there are quite a few Pokemon at the bottom of the NU barrel that would qualify as the most awful. E.g. while a lot of People argue that Pachirisu/ Emonga are just cheap electric squirrels made to manufacture cute plush dolls, I would actually argue that they had a use (mostly due to their DW abilities).
Castform, Chatot, Delcatty, Delibird, Spinda, Sunflora and Wormadam are all considered pretty useless, though Spinda is seeing more use with Contrary and Superpower.
Unown is probably the most useless Pokemon in the Metagame (it's not technically a first evolution since it doesn't evolve). On Smogon, at least you can pick your Hidden Power, but in reality, you're most likely to be stuck with a useless HP type. Even choice spec'ed maximum special attack is mediocre, and since you only get 1 move, all you need is a Pokemon immune to/ resistant to that move and Unown is dead. Coupled with a base speed/ defence/ sp. def of 48 that is liable to make you cry, Unown should never be used. Ever.
At least Castform has base 70 stats all round. Though having said that, Castform shouldn't be used for any reason apart from pity.