(Note: this is based on doubles, not singles)
Gen 1:
I think it's Charizard due to it's dominance when it got it's claws on it's megas and G max form, but even base form Charizard deals stupid damage under the sun.
Gen 2:
With Typhlosion's new hisuian form, it's the best because of those insane eruptions coming out.
Gen 3:
I'd say Blaziken, it's mega is good, it's damage is great, it's ability is insane, it's just a solid damage dealer.
Gen 4:
It's Infernape (I swear this isn't biased towards the fire types). It's similar to Blaziken, though I don't think it's as good with it being lesser in most things than Blaziken.
Gen 5:
It's Serperior no doubt (finally it's not the fire type). Serperior's Contrary ability is just ridiculous with Leaf Storm, due to it RASING Serperior's Sp. Atk and Serperior being very fast, I also found using Swagger to be evil with it.
Gen 6:
Greninja is the best, but Chesnaut isn't too far off. I feel like the thing that gives Greninja the edge over Chesnuat is that there's Ash Greninja, so it's a bit more unpredictable.
Gen 7:
It's Incineroar, I don't think I need to explain this choice.
Gen 8:
It's Rilaboom, I also think this is self-explanatory.
Gen 9:
The best is Skelidurge, it's so bulky, and has a good mix of being a attacker and supporter, plus Unaware makes it even harder to remove.