Good question. And you could make an argument for several different answers.
If you combine playing and coaching, which seems a little unfair, then it's Fred.
So let's look at guys just based on their time as players.
Note: I know I'm missing people. You all can make your arguments for them. I'm just going on who I best remember and my thoughts on them.
Jeff Grayer - Based on stats alone, he could easily be called the best Cyclone ever.
And when you look at the team results while he was here, they were pretty darn good, especially for an ISU program that was at a low point before Johnny got here. A Sweet 16 and two other tourney appearances were great for ISU at that point in time. But when you look at the total wins and compare them to the guys who just recently graduated, and even the guys from Floyd and Eustachy's era, it comes up a bit short. Now that's no fault of Grayer's. It's more a snapshot of where the program was at that time. He likely made the team that much better at a time when they needed him most. I wasn't watching basketball at that time, so I don't know him as well as some will. Maybe they can shed light on just how special he was. I believe he had injury problems in the NBA which limited his career, though he stuck around for a while. Depends how much you want to factor in professional career to Greatest Cyclone.
Still, a very good candidate for Greatest Cyclone ever.
Fred Hoiberg - He had a stellar career as a Cyclone and a very nice NBA career. One of the best shooters to ever grace the floor. Three NCAA appearances as well and a similar number of total team wins to Grayer during his tenure. Big part of building Hilton Magic and his success with Johnny and Tim Floyd led to Tim's best years before Tim left for Chicago. Cyclone Legend of course, but Greatest Cyclone ever based solely on his time as a player? Not sure.
Now we get into the Floyd and Eustachy eras. Some phenomenal players, though not all were here for 4 years to accumulate All Time stats.
Marcus Fizer - One of the most dominant players I've seen play for ISU. He just physically manhandled opponents. If he had stayed healthy he could have been much better as a pro as well. And if he hadn't been on those terrible Bulls teams. His 99-00 season was just phenomenal and he's relatively high on all time ISU lists because of it, having only been at ISU for 3 years. Had the team not struggled his mightily his first two years, he could have been much more dominant in an all time sense. The Elite Eight season, which coulda woulda shoulda been even better was an all time great season to be alive and be a Cyclone. So that alone puts him high on the list.
Jamaal Tinsley - If you're talking about the most talented player ever to wear a Cyclone uniform, Tinsley should definitely be considered in the top 5. He EARNED the Big 12 player of the year in 99-00 by dishing every kind of assist known to man and scoring at a darn good clip as well. Only two years at ISU really limits his all time stats, but impact on his teams was monumental. Huge part of ISU winning the Big12 two years in a row. This was when I was at ISU so it holds an even greater place in my heart and memory.
Sadly the Morgan and McDermott years had a surprisingly large number of very talented players, but they were all wasted on underachieving and disappointing teams. I care about them, but they probably aren't the Greatest Ever
Enter the Hoiberg era. The most successful in ISU history. Lots of good players and thankfully, even better teams.
Melvin Ejim - I'm surprised more people having mentioned him as I know he's a sentimental favorite and a great four years at ISU got him second on the all time rebounding list. While he was a essential part of starting the Hoiberg era, I don't think he's the Greatest Cyclone ever.
Royce MotherEffin White - (Middle name may not be accurate) - Possibly at the top of the Most Talented ISU players list, his one season was so good it earned the troubled player the 16th overall pick. Unfortunately everything went South from there professionally. At ISU he will always be remembered as the best player on the court among all the Kentucky lottery picks in the NCAA tournament. He almost singlehandedly willed us past them. White was the only player in the nation to lead his team in the five major statistical categories: points (13.4), rebounding (9.3), assists (5.0), steals (1.1) and blocks (0.9) that season. One of the best single seasons in ISU history. Can you imagine what ISU could have done with 4 years of Royce White? Or even just one more year in a Cyclone uniform to propel the 2012-13 team through the stratosphere. Not the Greatest Cyclone ever because he was only here one year. Greatest single season Cyclone, quite possibly. His success validated Fred's transfer strategy and lots of other teams have jumped on that bandwagon now. That success translated forward for Fred's coaching career as well.
Georges Niang - His scoring and over all play were something very special. As well as his affect on the program and the wins because of it. His injury in the 2014 NCAA tournament game against UCONN showed exactly how valuable he was. With Georges, a close loss could likely have been a win and an Elite 8 or better finish. Not a lot of players have had more impact on a program than Georges.
Monte Morris - Monte may be one of the few who can rival Georges impact on the Iowa State program. His assists records are amazing and he's also the steals leader. One of the best pure point guards to ever play the game in my opinion. Definitely in Iowa State history. Also the winningest player in ISU history with 100 wins.
Recency bias aside, I think Georges and Monte are the finalists for the Greatest Cyclone Ever title. I give the edge to Georges because of his overall impact on the program, but he couldn't have done it without Monte for three of those years.