So many different thoughts on athleticism here. For me it is that person who looks graceful, fluid, explosive, (and a host of other adjectives) while making it all look easy and effective.
As we grew up and played sports, we all knew that kid who ran like a gazelle with explosive, spring like steps. Looked graceful on the basketball court even though they might not have played on a competitive team. Made every motion look effortless even when playing sports they did not spend a lot of time playing. In my teens I played hockey with a kid who was the best basketball player in his high school (but stopped playing when he went to high school), crazy good at lacrosse and got D1 looks, great hockey player and went onto play D1, could do one and a half flips off of he side of the pool and still has the record at his high school for 400m run.
My gut tells me that most NHL players were that kind of kid. My vote would be Stamkos since the stories during his draft year centered around his skills at soccer and lacrosse along with hockey.