I wouldn't say this is THE worst Pokemon ever, but I've read through this thread some and want to be original, so here we go.
Heatmor - the Anteater Pokemon. In BW1, It's found at Victory Road,
right before the end of the main game, and I'm guessing most of you who played through BW1 might not even know this Pokemon existed unless you read Durant's Pokedex entry in Black version, or if you battled the
one mook in the area who has it - Veteran Tiffany (and I'm pretty sure you can walk around her too). To add insult to injury, Heatmor is only found on the outside of Victory Road at a 45% encounter rate, which is admittedly the highest of the encounter table, but why would you want literally the
last (barring the version-exclusive Reshiram) Fire-type in the Unova Pokedex?
But maybe it's not all about availability. Look at Bouffalant, Durant, and Terrakion, all of which who can be found nearby and can be put to use against the endgame via typing (former two) or coverage (Bouffalant) despite how late they come. Can Heatmor overcome the lategame odds and still be formidable?
...The answer is sadly no, which is ironic because Heatmor's prey, Durant, outclasses it in pretty much every single way (even having the same BST with competent Attack, Defense and Speed and an actually good defensive Type in Bug/Steel, which is great for endgame and in general). But I digress: here are Heatmor's stats:
HP 85
Attack 97
Defense 66
Sp. Atk 105
Sp. Def 66
Speed 65
They might not seem
atrocious right away, but let's compare Heatmor to the Unova Fire types. Emboar more or less hits better or about as hard on both offenses (clearly winning in Attack and only losing in Special Attack by five points) and takes hits slightly better overall. Simisear is loads faster than Heatmor in exchange for having only slightly worse mixed attacking stats and defenses (and you get it so much earlier like the others). Darmanitan pretty much outclasses Heatmor in
every way save Special Attack and also hits like a locomotive (the extra 20 HP Darmanitan has make Heatmor's 11 points in both Defenses more or less moot). Chandelure loses in HP and Attack but wins in
every other stat and has a Special Attack on par with Reshiram along with fantastic dual STAB. The only Pokemon this thing is really outclassing is
Larvesta, but that's moot because you can still get Volcarona immediately postgame anyway, which is a monstrous Special sweeper at level 70.
Does Heatmor get any moves that are different from the comparable Simisear (who gets most of the same moves)? No, not really, unless you count the glorious Bug Bite, Slash, Stockpile combo and Snatch "must-haves." What about Abilities? Flash Fire and Gluttony - each of which are had by Chandelure or Simisear respectively (Hidden Ability is White Smoke-Torkoal does it better). You can get Flamethrower at Level 51, and give it some TMs like Shadow Claw, Focus Blast, Fire Blast and Dig, but why bother when Fire STAB hits only 4 (out of 28, meaning one-seventh) of all endgame targets super-effectively?
So you've got a Pokemon at the
very end of the main game with no redeeming qualities, low Speed, and one that doesn't even hit many of the few foes left super-effectively with STAB (if it gets a hit off, pretty much all of endgame will slam it). And in BW2, you can only find it postgame in Twist Mountain (at a 5% encounter rate if not Summer, in the latter of which it has a 20% encounter rate). The only other game you can find it during the main story (or heck,
any game at all without transferring) in is X and Y (post-Team Flare with a worse encounter rate from 5-10% on Route 18, so availability strikes once again) where it can beat the final gym and flop against the League (hitting a whopping 4/22 targets super-effectively there, with three E4 resisting its STAB). Does anybody care about Heatmor?
...Apparently Game Freak does, because in Gen VII Heatmor actually got a signature move: Fire Lash, a 80-power, 100-accuracy Physical Fire-type move that always lowers Defense by one stage when it hits. A cool move, but nowhere near to save Heatmor from being hot garbage both in-game and competitively (it's Untiered in SM and even
Eviolite Combusken outclasses it, so yeah).
So Heatmor is my original pick for Worst Pokemon Ever. It's not
horrible, but worse things have already been said (Unown, Spinda, etc.) and I can't see why you would want it over any other Fire type (Unovan or otherwise) sans Magcargo (lol).