Horry is the only player to make this list on the strength of his playoff performance alone. He was never among the top players in the league in any single season, and never even performed at an All-Star level, but he is the only player in the history on the NBA to win more than 6 championships without playing for the 1960's Celtics.
Of course, being on the winning team is not enough to make a player one of the all-time greats. In fact, Horry was a non-factor during two of his title seasons (2001 and 2007), so those rings did not factor into his appearance in the top 100. However, in the other 5 title seasons, he was a significant contributor, and that is the reason he's here.
Horry was drafted by the Rockets at #11 in 1992, and when Michael Jordan retired after Horry's rookie season, a huge vacuum was created, which was filled by the Houston Rockets. In just his second season, he was the second-best player on the championship team, helping Hakeem Olajuwon win his first title. The next season, Clyde Drexler joined the Rockets in a midseason trade, and that trade boosted the Rockets to a repeat title, with Horry still chipping in as the #3 player on the team.
The next season was his personal best as a pro, with a scoring average of 12.0, and following that "breakout," he was traded to the Phoenix Suns, who soon offloaded him to the Lakers, which was a fortunate move for him. He joined a team that had just acquired both Shaquille O'Neal and Kobe Bryant, and within a couple years he was back at the pinnacle, although in a much-reduced role. He came off the bench for the 2000 and 2001 title teams, then earned back his starting role in 2002, becoming the third-best player on the team that completed the three-peat.
After failing to 4-peat, Horry left the Lakers to join the Spurs and bring him closer to his family in Houston, and that was another good move for him, as he was a strong contributor to the Spurs' 2005 title, behind only the three long-time Spurs stars. He held on for a few more years, and hit a few big shots along the way, but his final title was a gift from his teammates, with Horry only contributing 4.3 points per game in the playoffs.
The thing that helped Horry the most is that he always performed best in the playoffs, beating his regular season averages in every single one of his title runs. He earned the nickname "Big Shot Rob" for his many important late-game shots in the playoffs, and that ability earned him a spot among the top players of all time.