If we're going with most impressive, I would go with Gretzky.
That 92/93 season is probably the number one season for over-inflated point totals; Selanne was 5th in scoring in the league, but there were over 20 players with 100+ points that season (within about 30 points of Selanne). Gretzky tied for first in the league in scoring and the 4th place scorer in the league was over 30 points behind him. Most importantly though, Gretzky was 3 or 4 years younger than Selanne in his rookie season.
Selanne putting up 76 goals is unreal, but I think Gretzky coming in as a teenager and leading the league in scoring, while putting up over 50 goals as well, is more impressive.
Selanne, because Gretzky technically wasn't a rookie.
"Trust me I'm an expert, I watched 13 rangers games on NHL center Ice this year through streaming." -Starburst
"I don't even understand what the point of all this arguing is. Are you guys hoping that the other side is going to have an epiphany and go 'Oh, OH! You're right, we ARE going to lose this series!'" -Crease
Gretzky's year was more impressive as the raw totals are deceiving. Yes Selanne had 76 goals in his rookie season, but that was a year with 5 different 60+ goal scorers, including Mogilny who tied Selanne with 76 goals. Mogilny actually missed seven games that year, so he likely would have surpassed Selanne handily. Mario Lemieux was on pace to score 97 goals if not for some pesky cancer. Pierre Turgeon tied Selanne for fifth place in scoring, and they were both well behind Lemieux, Lafontaine and Oates.
During Gretzky's rookie season, which featured four fewer games, he tied Dionne for the scoring lead, and only Lafleur was close to them. Gretzky was 31 points ahead of the fourth place scorer. Gretzky also won the Hart, while Selanne was behind several players. Selanne even had a better supporting cast that year than Gretzky did, although the Jets certainly weren't a great team. Both deserve credit for phenomenal rookie seasons, but Gretzky's was better.
Gretzky was a teenager. Selanne? He was 22. Gretzky won the Hart Trophy. Selanne? No Hart. Gretzky tied for the points lead. Selanne? 5th overall. As for their overall seasons...it's not really close.
When did I say that didn't happen?
To be eligible for the award, a player cannot have played any more than 25 games previously in any single season, nor have played in more than six games in each of two separate preceding seasons in any major professional league. The latter fact was perhaps most prominent when in 1979–80, first-year phenom Wayne Gretzky was not eligible to win the Calder Trophy despite scoring 137 points (the previous rookie record at the time being 95), because he had played a full season the previous year in the World Hockey Association.
Now thinking about it, how did Selanne win the Calder under those rules? Did they not consider SM-liiga a major professional league?