Landorus-Therian #1: Landorus-T is undoubtedly #1 to me as it did nothing else but dominate. I remember the first months of SM and Lando went absolutely ham, there was a lot of discussion if it would finally be banned in this generation. Didn't happen because it provides not only offensive pressure but defensive glueing abilities as well and thus is a balanced pokemon that keeps other pokemon in check while being able to be checked. It is and was the face of offense and bulky offense throughout the generation, sets ranging from SSSS and Rockium, sd, rock polish and offensive rocks that let landorus itself shine to options like knock off, toxic, hp flying/hp fire/hp ice, stone edge, u-turn, scarf, rocky helmet, leftovers, defog, explosion sash leads, even the abr protect set and strong coverage move options like superpower and punishment, that really enable landorus to let the other 5 members of your team shine. Nothing else does as much for the metagame as this pokemon.
Magearna #2: Magearna is #2 and I doubt anyone would argue there. It's the most dangerous mon in the tier and is probably closer to getting suspected than landorus ever was. It's formed the metagame since the beginning with a lot of different sets, #2 is the only option.
Zygarde #3: Zyarde was in OverUsed for a good 2 years and the metagame revolved around it. It restriced teambuilding so much that you were forced into very standard patterns. It's plethora of sets decided if games were won or lost at turn 1. Seeing how much time it spend in the ou club I think it is well qualified to be the #3 even though it wasn't in the tier for the entirety of the generation.
That much on the top3. The rest is ordered by how big the influence was and how much time was spent in the tier. #10 was too close for me to choose. bulu koko and celes have been used in unbelievably many teams and kart joined a bit later but in return formed the meta a bit more on it's own than the others so it evens out. I'll be looking forward how the list of the ou forum turns out in the end and if it's gonna be any similar to mine.
Happy posting