The debate on which is the "best" chess website tends to be subjective and thus is inconclusive. On the other hand, the site that has the strongest players should be objectively ascertainable provided metrics are available.
So which is the strongest chess website in terms of:
ICC and playchess.com are definitely the top two in terms of the number of registered titled players who play actively.
I was able to find metrics on how many titled players are on ICC, seen here; I couldn't find the numbers for playchess, however they are similar. Many titled players play on both.
As of 2020 I believe by far the most popular sites to play online chess are lichess, chess.com and chess24.
I am not aware of any publicly available statistics on the metrics you suggest. From what I see many of the top players are active on several sites.
The commercial websites chess24 and chess.com are likely paying some of the titled players giving them an advantage. Still, even Magnus Carlsen is seen regularly on lichess as well.
On the other hand, what should count most would be, how likely is it that you play a game against a titled player. If that is your aim, the total number of titled players is fairly irrelevant and you'd better look at what tournaments/shows/... the websites offer in this respect.
Banter blitz sessions are one way, but there are also tournaments (particularly those with minimum rating requirements) that are a good way to find titled players.
chess.com is definitely one of the contenders. If you're a member, you'd know that GM Carlsen, GM Caruana, GM Nakamura; to name a few, are quite active there.
The question was which site has the strongest players. I presume this means rating to strength ratio. ICC is the leader there in my opinion, you really have to know your chess to get a good rating; it's a proper 'club'.
