Who is the best legendary pokemon is the best

There is no possible way to determine which is best,due to the fact that all have different stats and were trained by different trainers.At least before,there were no particular ways to defeat any specific legend.Now,however,I theorize that with the new generation of games and new pokemon came New pokemon whose sole purpose is to oppose a specific other pokemon.Take Mega Diancie for example.Almost useless,except to take out any particular Mega Rayquaza.Now,if you will give me your preferred battling style (offence,support defence,mixed defense,healing,sacrificial)and current team,I could build you a well rounded team good enough to handle almost anything.Post your response here.

All of them are in their very OWN unique, special ways ;)