I can say only that: all such comparisons are very subjective, criteria-dependent, opinion-based, and mostly, doesn't supported by facts.
Chili Peppers and Metallica are completely different bands, acting in different, non-crossing music genres.
I hardly can imagine how can they be compared objectively.
About the best band - yes, I can go that far by saying that they are the one of the most influential band of the last decade, at least for me.