I just finished a massive deep-dive into rating aggregators like AniList and MyAnimeList, partly because my friend won't stop recommending things. The lists are fairly predictable but still interesting. The usual heavyweights like 'Fullmetal Alchemist', 'Monster', and 'Vagabond' consistently dominate. Their averages are insane, often hovering above 9.0. It's the completed classics, the ones that had a definitive, satisfying ending, that seem to lock in those top spots.
That said, I've noticed a trend where slightly older, deeply atmospheric series get a huge boost from dedicated fans. Take 'Oyasumi Punpun'. It's not for everyone—it's brutal—but its rating is astronomical because it resonates so profoundly with a specific audience. The rating reflects a very specific kind of consensus, not just broad appeal. I find myself trusting the high scores on these denser works more than the scores on mainstream battle shonen, even the great ones.
Sometimes the highest-rated series is the one you've been avoiding because the summary doesn't grab you. I finally caved on 'Planetes' after seeing its score for years, and the rating was absolutely right. It wasn't what I expected, but it was perfect.