This question used to drive me crazy because 'highest ratings' is so vague. Are we talking Goodreads averages? Number of ratings? Professional critic scores? I eventually figured out you need to pick a specific platform and search for curated lists. For just raw, unfiltered reader opinion, I don't think you can beat Goodreads Listopia. Searching something like 'Best Fantasy Series of All Time' and then sorting the list results by 'average rating' gives you a crowd-sourced ranking that feels genuine.
Otherwise, if you want something more curated, 'best series' roundups from places like Esquire or Paste Magazine often have solid reasoning behind their picks, though they reflect one critic's taste. For me, the real trick is to find a reviewer on YouTube or a blog whose taste aligns with yours, and see what they've ranked highly. My own top series—things like 'The Stormlight Archive' or 'The Expanse'—rarely land in the same order on any two lists, which kind of proves the whole point.
Yeah, aggregation sites are the only practical way to get a consensus, but you gotta check the sample size. A series with a 4.8 average from 500 voters is less meaningful than a 4.5 from 200,000. I default to Goodreads for that exact reason, you can see both numbers right there.