There is none. I'm sorry, there just isn't a "best" for every single thing in the game as you seem to believe based on all the questions you're posting. Each Pokemon may be incredibly helpful on one team, but redundant on another. Some are better than others, yes, but I'd be wrong to choose one and say its the best.
We can use base stats as a rough indication of how powerful a Pokemon is though; it that's what you meant, then as of X and Y, Mega Garchomp and Mega Tyranitar have the highest base stat totals of all Pokemon at 700. Exclusing Megas though, this goes to Slaking, with 670. However, Slaking is a prime example of how base stats aren't 100% reliable; Slaking is a horrible Pokemon due to its ability Truant, which ranks it among some of the worst Pokemon.