So based on the facts:
1: REO - Indisputable. Won more events, and more major events, than any other player in MK9. Longest win streak of anyone in MK9. Has defeated more top competition than anyone else, and pushed his character to a level that no one else even came close to reaching.
2: CD Jr. - Although recent events have put Jr. farther outside the winners podium, he still holds the second best tournament record in the game. And in a lot of the tournaments where he lost, he was in Grand Finals. He was one of the only players in the game to ever give REO significant trouble or have a chance of going 50/50 with him.
3: Maxter - Outside of REO, one of the most consistent players in MK9 history. Almost always in grand finals or close to it, and leaves a long trail of defeated top players. He was a true threat to anyone he played and rightfully deserves to be in the top 5 all time.
4: PerfectLegend - Won EVO twice.
5: Dizzy - Dizzy has accomplished what a lot of people did not think was possible; he took a character that most people considered unlikely to win against the absolute top of the cast, and continued to refine him until he was capable of knocking off some of the best, no matter which characters they were playing. Defeated PL and Jr. to win Season's Beatings and elevated the game to a high art in his UFGT win. As the only person to get anywhere with Johnny Cage in recent times, in a world where everyone was flocking to Kabal, Cyrax, and Kenshi, I would have to say that Dizzy represents the highest level of achievement in Mortal Kombat skill.