Real GM has been hosting a top-100 players of All Time thread that started last August. The methodology is that a group of active posters got together, and each nominated a candidate for #1 with a justification for why. After the list was created, they took 2 days to vote for #1, and each person that voted would nominate a person for #2 with explanation. And so on, down the rankings. They are up to #68 of all time now.
This ranking list is far from infallible (I disagree with quite a few, myself), but it is a list where a lot of thought went into it from a bunch of relatively non-partisan fans. "Top player" rankings were done on career achievements, not necessarily a pure question of "who's best?" which is why LeBron is quite a bit further down than some of the others. Current players in the top-50 of this vote: Shaq (#8), Duncan (#9), Kobe (#18), KG (#20), LeBron (#36), Kidd (#38), Nash (#39), Dirk (#41), Iverson (#50). Current top-20 of the vote:
1. Michael Jordan
2. Kareem Abdul-Jabbar
3. Bill Russell
4. Wilt Chamberlain
5. Magic Johnson
6. Larry Bird
7. Hakeem Olajuwon
8. Shaquille O'Neal
9. Tim Duncan
10.Julius Erving
11.Jerry West
12.Oscar Robertson
13.Moses Malone
14.John Havlicek
15.Karl Malone
16.Bob Pettit
17.David Robinson
18.Kobe Bryant
19.Walt Frazier
20.Kevin Garnett