20. Nate Thurmond
Nate Thurmond is one of the greatest Centers in NBA history. This is not even debatable. He was also one of the greatest rebound artists in the history of the league as well. From 1963 to 1975, he averaged around 11 to 12 rebounds a game for each season. He had two years where he averaged over 20 rebounds per game too!! This is on top of a few years of averaging 17, 18, or 19 per game.
He was a great scorer too. From 1964 to 1974, he had at least 16 points per game. For 5 of those seasons, he put up 20 or more points per game as well. At one point, he pulled down 42 rebounds in a game. This is something only Bill Russel and Wilt Chamberlain bettered.
Nate is also credited with being the first man in NBA history to record a quadruple-double in a game. This only proves how amazing this all-around great was. Sadly, he never won the big one. Thus making him one of the greatest players who never won an NBA Championship.