In third place we have Incineroar. At first, second, third, fourth, fifth, and maybe even sixth glance, this Pokémon looks awful, but this Pokémon has way too much going for it for its own good. Incineroar is not a legendary, unlike every other Pokémon on this list. It is also a puzzle that we will solve one piece at a time starting with the biggest piece.
Incineroar's ability is the biggest piece to answering this puzzle. Incineroar has the ability Blaze. Blaze is a terrible ability on almost every Pokémon with this ability including Incineroar. One perk of not being a legendary is that you are much more likely to have more than one ability, and that is what Incineroar has. Its second ability is Intimidate, one of the best abilities ever. Intimidate lowers BOTH target's attack stat whenever the Pokémon with Intimidate enters the battlefield. This means that Incineroar has a large emphasis to switch in, so most Intimidate Pokémon want to have good bulk and typing so they can survive. This is why Pokémon like Mightyena, Masquerain, Stoutland, Overqwil, and Squawkabilly aren't good. As you may have noticed, Incineroar is not on that list.
Incineroar at first glance, doesn't look the bulkiest, but that is because you were just looking at the base stats of legendaries, and when you compare these to normal Pokémon, and the bulk is especially good, and it also has a good attack stat. It's special attack doesn't matter and even though it looks like it has bad speed, making it not as good, the low speed is actually a benefit, but you will have to remember that for later.
Incineroar has the type combination of Fire/Dark, which means that is is weak to Water, Ground, Rock, and Fighting. Fire removes Dark's weakness to bug and fairy at the cost of a fighting weakness, so you may think that the dark type just gives it an immunity to psychic and a weakness to fighting, but you would be wrong. Dark types have one special feature where they are immune to prankster.
Incineroar so far has been all over the place, but this is where everything from this Pokémon comes together
Incineroar is an amazing pivot Pokémon, which means it loves to switch in and out. We already know switching in activated Intimidate, but it also activates Fake Out, which is a 40 base power normal move that has +3 priority, meaning it always outspeeds other attacking priority moves and it also outspeeds redirection moves such as Follow Me and Rage Powder. Fake Out also always flinches, so the opposing Pokémon won't be able to attack on that turn. It also gets U-Turn and Parting Shot, which makes it want to switch out because U-Turn deals damage then switches you out while Parting Shot lowers the opponent's attack and special attack and then switches the user out. Flare Blitz is a 120 base power move that deals recoil damage and Knock Off is a 65 base power move that removes the held item of the target. Taunt makes the target not able to use status moves, Will O Wisp burns the target, also cutting their physical attack in half, and Helping Hand boosts the partner's damage by 1.5 times. This makes Incineroar have one of the best disruptive movesets ever and it has a lot of incentive to switch in and out, making it great with other pivot Pokémon like Rillaboom.
In my opinion, there is a clear divide between #1-3 and #4-6. The top three Pokémon have everything they need going for them, but #4-6 are amazing, but they may not have a tool that could make it better. (An example is that #4 could get Nasty Plot, but it doesn't.) The reason Incineroar is the lowest of the top 3 is that it is a support Pokémon. If it is the only Pokémon you have at the end of a game, then you will probably lose because it is not the greatest offensively even with its attack stat.
Even though this Pokémon is the best support Pokémon ever, Game Freak decided to buff Incineroar to get Parting Shot in Generation 8, a better U-Turn, and Helping Hand in Generation 9. This Pokémon is good, and probably isn't leaving anytime soon.