The best of what ? You haven't seen what's gonna happen TOMORROW. :)
Aw, c'mon Champ! The bell hasn't gone for the last round yet ;o)
It's not a bad attitude, but, it implies that nothing more is left to see or experience--that part of it is negative.
It's not bad, but it's not good either. No one has experienced everything. So no one can say they've seen the best of everything. You could live until your 200 years old and still not have lived long enough to know.