Your favorites I presume.
I've already bought down Charizard 100 times, you refuse to listen so let's move on.
Sceptile had a losing streak resembling Torterra's, possibly worse than Torterra's, at one point. It lost to a freakin' Luvdisc. I suppose they'll be some loop hole like "it wasn't completely evolved" but I don't care, it's the same pokemon. Considering you keep on bringing up game stats and type advantages when it comes to Infernape (even though it actually WON those battles) a pokemon actually LOSING to something that's widely considered one of the weakest pokemon in the game that it also had an advantage against should be grounds to be considered "the weakest pokemon ever" by you. It also lost to Greta without a single win and a pokemon you just said "wasn't that great" (Snorlax) destroyed both of her pokemon with a type disadvantage to boot. Bias is obvious here. It's win against Darkrai was undeniably pretty lame. You always go on about wins being handed to Infernape. Yeah, how about if a pokemon takes down a legendary in one hit? Don't even care that Gible and Heracross landed hits, that's still only three hits. That's less than what it takes to bring down most Gym Leader's pokemon. Let's review it's battles as a Sceptile, because I have a feeling you'll throw some excuse about those battles being in it's early stages at me. It lost to Regirock, we can't just assume that it would have won had Ash not been possessed. Possessed or not Brandon could have used Lock-On and there wouldn't be a damn thing for Ash or Sceptile to do about it. It won against Claydol, but it had a type advantage and we ALL know that those battles don't count, right?
It was matched with Blaziken despite a type advantage but so what? May was a coordinator not a battler. Sure her pokemon were strong against other coordinators but she never had a chance to prove herself against a trainer prior to that battle. And being evolved to the final stage doesn't necessarily mean almighty, unrivaled power. I'd say Bulbasaur is still about even with Sceptile.
Swellow... Well Staraptor defeated one of Paul's pokemon. Too bad people would rather assume that it was a "weak" pokemon dispite Paul's entire character screaming the exact opposite of any of his pokemon. Noctowl seems to be up to their level as well. It got raped against Latios just as badly as Torkoal did against Darkrai. Ash vs Tate and Liza was DEM as hell. So you, someone who detests Blaze (a legitimate ability from the game) should find this "Thunder Armor" tactic flat out ridiculous. It beat Venusaur but it had a type advantage, doesn't count
It lost to Roselia with a type advantage and it belonged to a coordinator. It also lost to Machop who was unevolved and it had a type advantage against it
Gible... Gible? Why is this even on the list? Because it evolves into a pseudo-legendary in the game? Because it landed one hit against Darkrai and then died? I genuinely don't get it. It did spectacularly against Conway defeating two of his pokemon and whatnot, but what pokemon didn't accompish that? Corphish defeated two of Flannery's, Kingler defeated THREE of Mandi's, Bulbasaur defeated two of Brandon's, Squirtle defeated two of Macey's, Buizel defeated two of Wake's, and Infernape defeated two of Fantina's, Byron's, Candice's, Volkner's and three of Paul's. Not that impressive when weighted against Ash's other pokemon.
Pikachu is variable. It can be Ash's strongest or weakest pokemon. Right now it seems to be weaker than Mijumaru and Pokabu. Maybe even Mamepato and Tsutaja.
And Heracoss is indeed a powerhouse, but still nothing special. A lot of pokemon take super-effective hits head on and survive. Grotle for example, took many ice attacks and still won. Squirtle took a few electric attacks and still on the battle. Cyndaquil was plunged in a freezing pool of water and survived. Defeating Nando's Kricketune is no more special than defeating his Roserade (apparently his starter) or Armaldo (Surprise powerhouse and a fossil pokemon). And landing a hit against a legendary does not make a pokemon strong. If that's the case than Torterra should be considered immensly powerful for landing a hit against Berta's Hipopowdon and and the same goes for Buizel for it's battle against Lucian. And then there's the movies. Croagunk stopped one of Dialga's Draco Meteors. An extremely powerful attack from a world-shaping legendary. That thing must have been the mosiah.
Power fluctuates, your favorites are not the best.