Personally, I think the worst Pokémon ever is a big group: Mythicals (and some would say that's grammatically incorrect, but Pokemon is both the singular and the plural, so there). Hear me out, please.
Mythicals have always annoyed because (aside from VC Crystal Celebi, ORAS Deoxys, and the infinite QR Code Magearna) Mythicals are IMPOSSIBLE TO OBTAIN (outside the event, of course). You can trade for them, but they always seem to be hacked, don't they? You can hack them, but then they'd be hacked. You can glitch them, but that's still kinda cheating. Since the beginning, I've hated Mythicals not because they were bad (bad design, bad movesets, which is the reason for most of the posts here), but because they were GOOD. I hated that, because I was denied the opportunity to obtain legitimately powerful Pokémon for a reason other than their rarity in the wild, and I've accidentally found a Riolu on an early Kalos Route and quickly found a Ralts in my second playthrough of Ruby, plus a bunch of others. I have no problem with that. And also, even when I do get them, I can't usually use them until postgame, since they're so high-leveled. Also, all of the Mythicals (most of them at least) look like great additions to your team, like they could be used early on. Celebi looks like one of the "Nostalgia for one of the first Pokémon I got in the game" type Pokémon. So does Mew. Meleotta looks like that late-game Elite Four wincon, and Victini looks like one of the cute-powerful first-route hard-to-find 'Mons that help later in the game, like Ralts. They all seem like they would be easily accessible and fun to use, but NOOOOO. Just had to be an excliusive event. Mythicals just infuriate me whenever I think about them, and I wish they were obtainable in more special in-game events like ORAS' Delta Episode and the GS Ball side quest. That'd make it a bit more fun.
The WORST part about it is that...I love Mythicals! Victini, Mew, Zeraora, Meloetta, they're all super cool! But I just can't get them! Most of the time, I just missed an event; worst one, though, is a story about Zarude. I got the code for it (thank you nintendo, that is so much more convenient) and I did not have Pokemon Sword. I got that code in December, and watching it expire over four months was super painful. I don't really like Zarude anyway, at least design-wise, but...ugh.
Thus ends my rant.
EDIT: Also, a lot of people seem to hate Miltank here. I personally love Miltank. It's a really good bulky physical sweeper and has a lot of cool moves, being a normal type Pokemon. Also, its Shiny is a blue cow. I see this as a hidden Star Wars reference, and you can't convince me otherwise.