That is a matter of opinion...
But seeing that I have one, I have to agree!
I have a stoutland and its my favorite non legendary Pokemon but Reshiram and Zekrom both are the strongest Pokemon on black and white. This is the link to see the srongest and fastest ETC.
Arceus
lv 100
At the Pokemon league.
Arceus the alpha Pokemon, it created pokemonand its base stats is the highest over any other Pokemon, Its the best Pokemon on paper, but when you play with it its not the BEST Pokemon out there. If you level all Pokemon to level 100 then Arceus would be the best, at level 100 it learns the move jugment. Also, if you catch a high level Pokemon, like a level 70 lugia, then level it up to 100, it wont be as stroung as if you got a level 20 lugia and level it up to 100, this is because the Pokemon earn more stats when you train them and not as much in the wild. So its best to get slightly lower level Pokemon, not the highest. Now, the best Pokemon in Pokemon hh and ss. This is only the best one avaulable to catch, without cheats or trading. The best Pokemon is probobly Lugia.
Arceus or rayquaza
level 100
a level 100 graveler
it is scisor
Rayquaza
Zekrom level 100 Arceus level 100 Kyurem level 100. Transfer Giratina level 100 dialga level 100 azelf level 100
the best way to get a shaymin level 100 is to get action replay max or rare candy and beating the Pokemon lege