I think 888 has better players that Pokerstars, but it's hard to define, because most of the people who play at Pokerstars also play at 888, the only difference is that in Pokerstars you could fine more beginners that are easy to beat
in my understanding the most consistent players are on ACR for example or on other sites that allow American players, I think the USA is the most traditional country in poker, I am Brazilian in my country, there is no tradition in the game of poker, casinos are prohibited, in the online game there are already a lot of people trying to learn to play poker
i cant say how strong the players are on the sites that dont allow USA competitors but there are lots of strong ones on ACR and Betonline. Everygame (used to be 'intertops') has lots of poor regulars, but the volume is so small its hard to find much going on, in SNGs anyway)
the pool of players is almost the same every day is played by the same players as 888 Poker and pokerstars, the best players are those who make it to the final table and there the winner can be anyone, and such players change every day can reach the final table another player and win the tournament so we can't find out who is the best for every day, but this evening is considered the best player in 1st place