For me, it's: Sakic>Mackinnon>Forsberg.
Sakic for me is the best out of these 3 because he finished with 118 points (22 points ahead of 3rd place) and simultaneously finished 2nd in selke voting during a lower scoring era than Mack. Imo this season is up there with Fedorov's Hart in 94; the competition in 01 isn't quite as high when you compare the top 10 leading scorers from both seasons, but the margin of separation is still pretty ridiculous between Sakic and the rest of the top 10 and he's pretty much scoring just as much as Fedorov that season in a lower scoring era. This is the greatest regular season campaign in franchise history as far as I'm concerned.
While Mackinnon isn't a stellar two way player, I think this season is probably the highest offensive peak out of the 3, albeit, probably not that much better than Sakic's. I could probably switch between him and Forsberg but if I had to pick today, I'd go with Mackinnon in 2nd.
Forsberg in 03 is phenomenal also, but like
@Hockey Outsider stated earlier, his all around game wasn't as potent at this point in his career as it was earlier so I can't put it above Sakic's and I don't think it's decisively better than Mackinnon's. Naslund isn't quite the competition that Kucherov is (He did go up against Thornton who was coming into his own though, but Thornton in his prime isn't on Kucherov's level at their peaks IMO).