Let's get something out of the way to start with: I'm more than a little biased. I've watched Mark Stone play hockey all season. I'm a passionate fan of the Ottawa Senators. Still, I think Stone has the strongest case for winning.
First of all, scoring is often used as the benchmark for the Calder winner. Stone and Johnny Gaudreau tied for the rookie lead with 64 points. Stone tied with Filip Forsberg for second in rookie goals with 26, one behind Stone's teammate Mike Hoffman. Stone also led rookies in even-strength assists (30) and was second in even-strength goals (20). Offensively, Stone deserves the award.
Some point out (truthfully) that Stone rode a hot second-half of the season to his nomination. I counter by saying that Stone started putting up more points as he started to be used more effectively. In the 2014 portion of the 2014-15 season, he played 15:09 per game. Remember, this was also when the Kid Line existed -- Stone with Hoffman and Curtis Lazar. 17 points in 34 games isn't bad, but Stone is capable of a lot more. It was a reflection of his ice time with essentially an energy line. However, under Cameron, he really started to come into his own. In 2015, Stone put up 47 points in 46 games, averaging 18:24 per night. Stone also started to play with Kyle Turris, and either Clarke MacArthur or Milan Michalek. Stone's abilities had earned him a shot on the top line, with top-line teammates. His production increase reflected that change. In 2015, Stone scored 32 points at 5v5, which was first among all hockey players (Art Ross winner Jamie Benn came second with 30). Among players who played at least 100 minutes, Mark Stone's 3.08 points per 60 minutes of 5v5 again put him first among all players. Stone wasn't only pacing rookies - he was pacing the entire league. Some would say that Stone's hot finish to the season was unrepeatable. I'd counter by saying that Stone's finish to the season was a clear representation of what he can do when deployed properly as a top-line player.
The other place that Stone shone was on the defensive side of the puck. His 98 takeaways tied him with Ryan O'Reilly for the league lead -- the first time ever a rookie has done this since the stat's been recorded. Stone was a positive possession player, and improved nearly everyone on the team. Erik Condra and Matt Puempel were the only players on the team to put up 5v5 Corsi below 50% with Stone and above 50% without him. Condra played only 47:29 with Stone at 5v5; Puempel, only 3:20. Corsi-wise, Turris performed worse without Stone than Stone performed without Turris. With Michalek, the effect was even more pronounced. In short, not only was Stone a positive possession player, but he appeared to be driving the bus on his line.
If you're not much into the advanced stats, I'd suggest looking at his coach's trust. Cameron came to trust Stone more and more down the stretch. From March 31 to April 7, there was a five-game stretch in which Stone topped 20 minutes each night. That was highlighted by a game on April 7 against the Penguins, in which he played 24:23, more than a minute more than Marc Methot. In the modern NHL, forwards don't play more than top-pairing defencemen. Yet with his team losing, Cameron kept going back to Stone (and Turris) for the eventual win.
You could make a strong case for Aaron Ekblad to win, but most of the arguments end with "for an 18/19-year-old". Last I checked, the Calder isn't handicapped based on age. Ekblad had a solid defensive season paired with a great mentor in Brian Campbell. He looks to be a mainstay on the Panthers' blue-line for the next decade and a half. Still, his season was arguably less impressive than rookie defeceman John Klingberg's. Klingberg had less hype, and put up more points in fewer games while doing more to drive possession. But Klingberg didn't play the whole season, and is a few years older, so he got snubbed.
Johnny Gaudreau I don't think belongs in the same conversation. Gaudreau had a great rookie season, and his combination of skill and smarts look to make him a great player for years to come. Still, he only got to this year's highs by playing on a Flames team that performed far better than they should have, and by playing on a line with a career-best Jiri Hudler. The 31-year-old Hudler scored 19 more points than his previous career high. I don't think it's fair to say Hudler grew tremendously from last year at his age, so it's more likely that Gaudreau benefited from a lucky up-year by the veteran. Possession-wise, Johnny Hockey had a dismal year, finishing below 50% in 5v5 Corsi, Fenwick, and shots for/against. Gaudreau is the kind of guy you put out late in a hockey game when you're losing. Stone is the kind of guy you put out late in a hockey game regardless of situation. To me, that sums up why Stone is the best candidate.
I don't believe in a player's ability to elevate their game at will, but at the same time, Stone is regularly put in positions to be a hero and succeeds. Here is a smattering of examples of his beautiful work from last season:
March 31: SO winner against Detroit
April 7: OT winner against Pittsburgh
April 11: Stripping Jakub Voracek of the puck, then scoring a goal to assure Ottawa's playoff berth
Really, whether you look at points, skill, analytics, hard work, determination, two-way hockey, or enthusiasm, Mark Stone is the most deserving candidate. I can only hope the Professional Hockey Writers Association did their homework and gave the trophy to best rookie of 2014-15.