With both Bryce Harper and Mike Trout getting the call to the big leagues recently, Dylan Bundy is now the official engineer of the Prospect Hype Train, and with good reason. He's faced 52 batters on the young season, and three have reached base, while 25 have struck out. That has prompted the inevitable questions—especially on Twitter—about whether Bundy can become a No. 1 starter. However, becoming a No. 1 starter takes more than just stuff, or more than just command; it takes something that is more than a little bit ineffable.

To be clear, “No. 1 starter” is a scouting/industry term, not a slot in the rotation. There were plenty of No. 3 starters taking the bump on opening day. There are maybe ten No. 1 starters in baseball. These are the guys who enter your head every time it comes time to predict who will win the Cy Young award each year. It takes stuff and command, but also durability, consistency, and that extra something else.

“The label is the ultimate of the ultimate, and there's nothing wrong with a strict standard,” said an American League executive. “I never project a prospect as anything more than a No. 2 starter for a reason,” he continued. “You're not a No. 1 until you actually prove over a period of time that you are a No. 1 in the big leagues. You cannot be anointed by a scouting report.”

“The radar gun can't tell you who is going to be a No. 1,” agreed an American League scout. “We envision guys in the mid-90s with crippling breaking balls and physicality, but at the end of the day, guys like Verlander or Pedro Martinez become No. 1s at the major league-level. Until you are pitching in the big leagues and putting away Cabrera and Fielder back-to-back, you're not a one.”

As for the extra component, people in the industry have trouble explaining it, but they know it when they see it. “No. 1 guys are the guys where every time they take the mound you feel you are going to win. Every time out, you are going to get a defined area of performance. Good starters will give you average starts half of the time, and then be really good 25 percent of the time and bad the other 25 percent,” another executive explained. “A No. 1 is giving you 50 percent elite starts, 25 percent really good and 25 percent average. When you see Verlander get hit hard by the Yankees, you know he's going to deal the next time out. That's a No. 1. When you see Sabathia going late in the season for the Brewers on three days rest and pitching them into the playoffs, that's a No. 1. Most starters can't do that.”

So while Bundy certainly has the makings for being a No. 1, he's far from joining that exclusive club. “The hyperbole is justified,” said a scout who has seen Bundy since his high school days. “His stuff and makeup are off the chart and what he's doing to start a professional career might be unprecedented. But it's a long journey from Delmarva to No. 1.”

Not only are No. 1 starters rare, so are prospects with even the chances to even be a No. 1 starter. When queried as to the numbers of even potential No. 1s in the minor leagues, the answers ranged from just five to fifteen. That's well under one percent of the professional pitching population with even a chance to become an elite starter, and most of their chances are slight. “There have been plenty of guys who looked like No. 1 starters in the minors or in the draft, and we've ended up with far more three, four, five starters or relievers than ones,” said a National League exec. “Whoever takes [Stanford RHP] Mark Appel in June is going to have a press conference, and they are going to say he has a good chances to be a No. 1 starter,” added an American League assistant GM. “And he does. But if we only have ten or so of them in the big leagues, what really are the chances?”

Understanding the rarity and the unlikelihood that any pitcher will become a No. 1 starter, somebody certainly will, and here are five players the industry sees as having the best chance.

1. Taijuan Walker, RHP, Mariners (Double-A Jackson)
For many in the industry, Bundy's performance hasn't even been the most impressive by a pitcher in the minors. Consider Walker, who has a 1.64 ERA in four starts while striking out 26 over 22 innings and limiting the Southern League to a .203 batting average. Those numbers certainly aren't as gaudy as Bundy's but Walker is doing it at Double-A, and he's just three months older than the Orioles' prize pitcher. With a 92-97 mph fastball, plus-plus mid-70s curveball and impressive changeup, Walker certainly has the stuff to be a No. 1 starter, and his ability to dominate at such a young age says something about his makeup as well. “Every pitch he throws comes out of the same slot and it's just impossible to pick up,” said a National League scout. “He's like a poor man's Doc Gooden, he's really out of that Mantra.”

2. Dylan Bundy, RHP, Orioles (Low-A Delmarva)
Bundy's numbers have been well documented, but he also has No. 1 stuff with an upper 90s fastball, as well as a curveball and changeup that are both already plus. “When you think about future No. 1 starters, Bundy is the guy who has a real chance,” said an American League scouting director. That belief was universal, but one AL executive was sure to point out that with just 17 innings of pro experience, there is still plenty that we don't know. “Bundy is obviously [expletive] ridiculous, but there are things he is still going to go through,” explained the exec. “He's going to get punched in the face, and nobody knows what a guy's makeup is until they fail.” The problem that Baltimore faces, according to the exec, is that they can't force that failure. “Bundy is not going to get touched in Low-A,” he said. “Short of starting him at Double- or Triple-A, there is no way to artificially create that challenge.”

3. Archie Bradley, RHP, Diamondbacks (Low-A South Bend)
The seventh overall pick in last year's draft, no 2011 pick has already done more to boost his stock. From stunning showings in instructs and spring training to a five start stint in the Midwest League in which he's allowed just eight hits over 26 innings while striking out 30, Bradley has shown the rare combination of elite velocity and sink to go with a curveball that earns just as much praise. “I'd actually take him over Bundy,” said an American League scout. “He's the more traditional physical package you look for in a future ace.”

4. Gerrit Cole, RHP, Pirates (High-A Bradenton)
Coming off his best start as a professional on Monday, when he allowed one hit over six shutout innings, Cole has a 3.54 ERA in five starts and 29 strikeouts in 23 innings, but his stuff is still way ahead of his numbers. “People forget that this guy throws 100 mph every time out, and has two plus offerings behind it,” said a National League scout. “If he figures out how to harness everything, he could be as good as any of them.”

5. Jameson Taillon, RHP, Pirates (High-A Bradenton)
The Pirates have focused on pitching with their top picks of late, and Taillon has begun to dominate following a 2011 season during which the Pirates often tied one hand behind his back by forcing him to go without his plus-plus breaking pitches. With the training wheels finally off, he has a 1.46 ERA in five starts with 28 strikeouts and just four walks in 24 2/3 innings. “From the body, to the frame, to the delivery, to the stuff, to the command, I'm not sure there's another guy who can check off as many boxes when it comes to finding a future No. 1 starter,” said one National League scout.

A version of this story originally appeared on ESPN Insider Insider.