I nominate Basculin.
Now, to explain why I think this is the worst pokemon ever, I have to get a little verbose, so bear with me.
Half the reason for me not liking Black and white was that a lot of the new pokemon looked derivative. Game freak had the awesome idea to make a new continent using only new pokemon, to provide a fresh experience. Shortly thereafter they also had the horrible idea to butcher the previous one by designing the new pokemon to look and function map-design wise, in the same ninches that were alrealdy filled by the old pokemon. I mean, look at Woobat and Roggenrola, for example. They are
clearly meant to replace, to some extent, zubat and geodude as the token bat and token rock, and were found in the same kind of places that you would find zubats and geodudes. Well, if you will use not!Zubat and not!Geodude anyway, just use the original ones, dangit.
Now, just because it was a bad idea from a game design standpoint, it doesn't mean that every derivative design was bad. We got a lot of good pokemon in gen V, much in the same way that we have a lot of crap in gen I and 2. Just to cite a few, Venipede is, IMO, the best three-stages bug evolution line in the entire franchise, Galvantula is awesome, Sawsbuck is a better deer than Stantler will ever be, the woobat line is mechanically different from the zubat line, and even the token pikachu clone had an ununsual type. If anything, gen V proved that you can instill some creativity even when you don't have a lot of elbow room.
This brings me to Basculin. This thing is most uneeded, redundant, derivative, useless, forgetteable heap of shit that ever bore the title of pokemon. There is no reason for basculin to exist at all. Of all derivative pokemon, it is the worst. There is not a single ouce of creativity anywhere in it's design. It's a green fish. That's it. There is no cool mechanic, no interesting type combination, no unique ability, nothing. Even the different forms gimmick was alrealdy done better, with Gastodon. The only reason people remember it at all? It's raison d'être is to be the generic water-type fish and fill spaces. It appeared EVERYWHERE, because it basically was supposed to replace the entire water 2 egg group. According to bulbapedia, Basculin can be found in no less than 14 areas in pokemon Black/White, and a whooping 26 areas in B2W2.
It isn't a joke pokemon either. Stuff like Luvdisc and Delibird were designed to be bad and are, in some way a success. They are not to be used seriously. And hey, even the joke pokemon get some uses. Delibird is one of the few pokemon that learns Rapid Spin, even if he sucks at it, so he got some rare use now and then, and luvdisc the main source of Heart Scales. Basculin? He has nothing. You can't even EV train properly by killing it, because after you get the power items, they provide 6 EVs per kill, which a very awkward number to keep track of.
It was overused but is otherwise completely forgettable, the design is beyond bland, has no distinguising features, doesn't work as a joke pokemon, serves no in-game purpose, and from a competitive standpoint, it is mostly outclassed even in NU. I can think of no better option for the "Worst Pokemon Ever" position than Basculin.
Edit: because I suck at english