Criteria for me is at least 100 GP in the blue & white this decade, and based on a mixture of peak & overall contributions. Tried to make line combinations that make a bit of sense, not just loading up the best offensive players onto four lines. That left me with this:
Kadri - Tavares - Marner
Lupul - Matthews - Nylander
van Riemsdyk - Grabovski - Kessel
Kulemin - Komarov - Hyman
Rielly - Gunnarsson
Gardiner - Franson
Phaneuf - Hainsey
Andersen
Reimer
The offense has potential to be terrifying. The first line in a matchup role against top competition would be great, and the third line in a sheltered, soft-scoring role would be deadly. The second line of course would be money as well, that Matthews/Nylander combo has been deadly, and a guy of Lupul's skillset would easily contribute. The fourth line is three guys who are great defensively, PKers, but at their peaks all had 20+ goal ability as well, and can be absolute pests.
Defense is a bit sketchy though. Franson is really the only right handed defender of any merit the Leafs have had (Zaitsev, Schenn, Polak, Komisarek, & Carrick the others). The pairing of him & Gardiner was a great combination, two of the most underrated players on the Leafs in the past decade. The problem is that, Rielly still doesn't have a partner...Gunnarsson as a stay at home, shot suppressing type seems like a better fit than the higher risk Phaneuf, who would still be a monster on a third pairing against weaker competition, with a solid veteran like Hainsey as his partner.
Andersen obviously shores up the goaltending, and Reimer can be a solid backup, as he is proving this year with Carolina, and had some incredible runs with the blue & white.
If we're going by some of my actual favourite line combinations of this decade:
Hyman - Tavares - Marner (2018-2020)
Johnsson - Matthews - Nylander (2018-2020)
Kulemin - Grabovski - MacArthur (2010-2012)
Komarov - Kadri - Brown (2016-2017)
Kaberle - Komisarek (2009-2010)
Gardiner - Franson (2013-2015)
Muzzin - Holl (2019-2020)