I'll leave it up to you to determine what "better" means. Stats may tell part of the story, but not the whole story. Stats, longevity, effectiveness, among other factors are things to consider.
I think Ennis. He has the speed, he has the creativity, he has the hands. He has enough size to survive the NHL game long term.
Gerbe at his intensity and size, I think he will wear down as he ages, and injuries could have a very devastating effect. Especially if he loses his speed. But if he's lucky enough to have a career as long as St. Louis, that would be cool to see.
Adam at his lack of speed, and current lack of physical play may turn him into a 3rd, 4th line guy in a few years. He has good instincts, and good hands. But he may get better and better and surprise me.
My guess would be Ennis, as I think he will be productive offensively for his career. Tough, though, for me to dismiss Gerbe. If he can play like this for his entire career, it'll be him with the "best" NHL reign.
No slight on Adam, just a personal hunch based on the question posed.
Gerbe has been awesome for about a full calendar year now. Love it.
Ennis has the skillset to be a top liner in the NHL, hopefully with experience the tendecies to "Afinogenov" the puck will dissipate a bit.
Gerbe's only true downfall is his size. But he makes up for it with tenacity and grit. The more he plays the better he gets, take that play in Ottawa for example....smashed into the boards, gets up grabs puck makes a play....that is what makes you successful.
Adam was hard to put below gerbe. He has the body, the hands and the vision to be an extremely successful player, and has shown flashes of being a two-way forward with his backchecking a few games back. My problem is he's slow and seems to run outta gas too quickly. In a few years if he's able to work on this he will overtake gerbe, and a longshot ennis. But right now, based on what I see, thats how I see them.
I think it's far from safe to assume that Gerbe, of that group, will lack for longevity due to injuries. He has a far better build than Ennis. I'd be much more worried about Ennis getting long-term injuries.
As for my answer, I don't really feel comfortable picking between Gerbe and Ennis, because there are so many X-factors to how Ennis could develop, and just how well Gerbe's offense comes along at this level is another big question. I'll go with Ennis, I suppose, which means I'm leaning more towards "best case scenario" here.
Gerbe will be most beloved by fans, even if he doesnt have the stats. Its his intangibles. (Peca, Kaleta, Ray) He is not getting lit up on any regular basis for body to wear down. In fact, I would predict that Ennis will be more brittle than Gerbe.
Have to see Adam play with other than our two studs to see if he gets the same production/chances. He may end up with the most goals.