Was it his arm or his instinct? Was it his accuracy or his competitive edge?
Raymond Berry, one of his favorite targets, says it was all of the above.
Unitas could throw the ball where only a receiver could catch it. He could intuitively make the right play call to exploit a weakness in the defense. He could stare down the pass rush and deliver the ball to the right receiver.
And always, he could beat the clock.
"I think he had the total package," Berry said yesterday. "[But] it takes more than the physical package to excel at the level he did.
"Intangibles are what really brought this all together. ... When you put it all together, the combination was unbeatable."
Unitas' death on Wednesday from a heart attack brought back vivid recollections of his toughness and his arm strength and his iron will, all legendary. What Hall of Fame coach Marv Levy pointed out, though, was Unitas' impeccable throwing mechanics.
"His mechanics were magnificent," Levy said. "He was the textbook: This is the way to drop back. This is the way to set up. This is the way to direct your eyes and head down the field. This is the way to hold your elbows in.
"His mechanics were unparalleled. The closest I've ever seen to his mechanics is Peyton Manning."
That Manning should play for the transplanted Indianapolis Colts is one of life's small oddities. That Manning should dress each day in the Colts' locker room under a 6-foot-by-8-foot mural of Unitas, leaning back on his heels as he's about to launch a pass in the 1958 championship game, is food for thought.
That's why every now and then, Colts general manager Bill Polian will point to the mural and deliver a history lesson to some of his younger players.
"I remind them about those that came before and what they did to put the NFL on the map," Polian said. "The '58 game in my mind was really the turning point."
Unitas was the hero of the so-called "Greatest Game Ever Played," directing a two-minute drill to get a tying field goal and then a patient, 80-yard drive to beat the New York Giants in sudden-death overtime.
Unitas may not have embraced the Colts' move to Indianapolis, but the Colts still embrace him.
"His place is secure among the greatest of all sports icons in American history," Polian said. "He transcends the National Football League or quarterbacks."
Unitas' legacy touches everyone who has ever played in the NFL. To James Harris, an NFL quarterback 13 years and now director of pro personnel with the Ravens, Unitas was a quick study in how to do it right.
"I thought he was a great player, very unique in that he didn't appear to have a great arm, great size, great mobility," Harris said. "Yet, he may have been the best of them all because of all the other things.
"He did so many things well in terms of managing the game, calling the plays, getting the ball to the right person. And when the game was on the line, probably as well as any, he produced."
Count Ron Wolf, former general manager of the Green Bay Packers, as one of those who thinks Unitas was the best.
"If you had to pick one guy, somebody who has to be the best, in my opinion, he's the best," Wolf said.
Wolf grew up in New Freedom, Pa., just across the state line. He grew up a Colts fan. And in 1957, while on leave from the Army, Wolf made the trip to Westminster to see the Colts in training camp. That was Unitas' second year with the team, and George Shaw's first year back from an injury that forced him to give up his job to the strong-armed quarterback from Pittsburgh.
"I watched practice," Wolf said. "And when Unitas was under center, it was obvious who the best one was. Unitas took command. He had that presence. Those people who do that have that presence."
It was a presence that was clearly distinguishable from the other sideline as well.
"There was a coolness and a constant 'in-control-of-the-situation, I-understand-what's-happening,' " Levy said. "There was never any sense of panic setting in, even in the most dire of situations.
"In order to make all of that work with a lot of great teammates around him, he had the magnificent balance between confidence and lack of ego. He's one of those guys who expected to succeed, but had no swagger."
Bruce Laird spent 10 years with the Colts as a hard-hitting safety, but only his first season was spent with Unitas in 1972. Being in Unitas' presence as a rookie was special.
"It was eerie, like being in the presence of the Almighty," Laird said.
"He wanted to be great. He came out, wanted the ball, expected to play well every game. He expected his teammates around him to have the same compassion and love for the game. If you didn't, he'd let you know in no uncertain terms."
Still, it was Unitas' ability to make the right call in the days when quarterbacks called their own plays that earned him a special place among his peers.
"He was like a coach on the field," Harris said. "He really knew the game plan and implemented it. He was a master at calling plays and making decisions. I wasn't in the huddle with him, but I watched him play and he appeared to be a master at going to the right guy at the right time."
Berry said that was the result of film study, dedication and a simplified offense under coach Weeb Ewbank.
"If you send a solider into battle with too many weapons, he will be all weighed down and not use the ones he's got," Berry said. "If you send him in with just a few, he can be really effective.
"I think John in that type situation was able to make the absolute most out of it and the system we had. He was not overburdened with too much and he was able to excel at it."
Mike Curtis, the Colts' All-Pro middle linebacker, credits Unitas with helping prepare him each week for the Colts' opponent.
"What helped me was his technique for completing the pass," Curtis said. "He was real good at faking the ball to the running back or looking [defensive backs] off. The other quarterbacks I had to face couldn't compare.
"He has a good touch short and a good touch deep. He did what he had to do. If he had to throw a bullet, he threw a bullet. If he had to throw a creampuff, he'd throw a creampuff."
Berry spent endless hours - after practice - working on his timing routes with Unitas.
"As a passer, there was no throw he couldn't make," Berry said. "Plus, he was so accurate. He could put the ball in places that were safe."
Unitas wrapped it all up in a package that set the standard for years to come.
"He had a great arm," said former Colts general manager Ernie Accorsi, who holds the same position with the Giants. "He had physical attributes. That's sometimes overlooked.
"If I said one of two things set him apart, it'd be his eyes and stomach. You looked at those eyes, those steel-gray eyes, and he came to beat your [butt]."
And he usually did.