These are all personal opinions, because it's really what you do with a pokemon that make them strong. You can easily make a non-legendary stronger than a wild legendery to the point you can KO one of them.
The pokemon I personally think is the strongest is Typhlosion. He is weak against ground, rock, and water, but can learn Solar Beam to be able to take them all out. He can also learn Earthquake to be able to take out various non-flying pokemon. With Sunny Day, he can fire solar beams without charging up, and it'll boost whichever fire based attack you decide to teach him. If he has flash fire, he'll be immune to fire based attacks, and will actually get a boost from one. Then with Blaze, he'll be able to increase his fire attack power even further. Combined with STAB in terms of his fire technique, Flash Fire, Blaze, AND Sunny Day he could potentially increase the power of his fire attack by 300%. If the attack has a power of 150, it would then become a 450 power attack. That would take down just about anything.
The only other pokemon that I think would be on par compared to Typhlosion would be a Breloom with a Toxic orb. Due to Breloom's special ability, it would actually gain HP from being poisoned and since it would be poisoned, it would not be able to get burned, paralyzed, or frozen. You can also teach it the tm move Facade, which would increase in power to 140 since he would be poisoned. Unlike most moves in the 140-160 range, Facade has a lot of PP and Breloom's above average attack stat complements that move well. Pretty good combination I think.
The pokemon I personally think is the strongest is Typhlosion. He is weak against ground, rock, and water, but can learn Solar Beam to be able to take them all out. He can also learn Earthquake to be able to take out various non-flying pokemon. With Sunny Day, he can fire solar beams without charging up, and it'll boost whichever fire based attack you decide to teach him. If he has flash fire, he'll be immune to fire based attacks, and will actually get a boost from one. Then with Blaze, he'll be able to increase his fire attack power even further. Combined with STAB in terms of his fire technique, Flash Fire, Blaze, AND Sunny Day he could potentially increase the power of his fire attack by 300%. If the attack has a power of 150, it would then become a 450 power attack. That would take down just about anything.
The only other pokemon that I think would be on par compared to Typhlosion would be a Breloom with a Toxic orb. Due to Breloom's special ability, it would actually gain HP from being poisoned and since it would be poisoned, it would not be able to get burned, paralyzed, or frozen. You can also teach it the tm move Facade, which would increase in power to 140 since he would be poisoned. Unlike most moves in the 140-160 range, Facade has a lot of PP and Breloom's above average attack stat complements that move well. Pretty good combination I think.