How would you rank the 5 on 5 stats? Its hard to do because lines don't actually exist in a vacuum, they work as units of 6.
It is hard to do, but I would start with ES goals scored by the three forwards together and +/- of the three forwards. Given that the Danault line played against top lines more than their share, this could give us an idea of their 5-on-5 value. No doubt the PP would be another matter, as neither Danault nor Gallagher produced on it. But we can't forget that the claim being tested is how good the Danault line was at 5-on-5.
So, for 2018-19, here are the simplified stats for ES play. I've totaled them by adding the ES Goals scored plus 1/3 of the +/-
93.3 = 76 G, +53 for Kapanen-Tavares-Marner
89.3 = 71 G, +55 for Stamkos-Point-Kucherov line
87.3 = 69 G, +55 for Gaudreau-Monahan-Lindholm
87.0 = 76G, +33 for Panarin-Dubois-Atkinson line
82.7 = 65 G, +53 for Benn-Seguin-Radulov
81.3 = 68 G, +40 for Landeskog-Mackinnon-Rantanen
78.0 = 62 G, +48 for Danault-Gallagher-Tatar
77.7 = 63 G, +44 for Marchand-Bergeron-Pasta
77.3 = 68 G, +28 for Guentzel-Crosby-Hornqvist
76.3 = 75 G, +4 for Draisaitl-McDavid-Chiasson
70.7 = 66 G, +14 for Ovi-Backstrom-Oshie
70.3 = 57 G, +40 for Vrana-Kuznetsov-Connoly (Connoly had better stats than Wilson)
70.0 = 61 G, +27 for Forsberg-RyJo-Arvidsson
69.0 = 52 G, +51 for Nino-Aho-Teravainen line
66.7 = 67 G, -1 for Couture-Kane-Meier
65.7 = 68 G, -7 for Saad-Toews-Debrincat
64.3 = 64 G, +1 for Connor-Scheifele-Wheeler line
62.3 = 61 G, +4 for Kahun-Strome-Kane
61.0 = 55 G, +18 for Pacioretty-Statsny-Stone
59.0 = 63 G, -12 for Giroux-Couturier-Konecny
58.3 = 49 G, +28 for Palat-Johnson-Gourde
58.0 = 50 G, +24 for Sundqvit-ROR-Perron line (Sundqvist had the 6th most ES goals of the F)
58.0 = 59 G, -3 for Bertuzzi-Larkin-Mantha
55.3 = 50 G, +16 for Marchesseault-Karlsson-Rielly
53.3 = 61 G, -23 for Huberdeau-Barkov-Dadonov
47.7 = 48 G, -1 for Jonnson-Matthews-Nylander
44.7 = 43 G, +5 for Schwartz-Schenn-Tarasenko
41.0 = 44 G, -9 for Goldobin-Pettersson-Boeser
39.0 = 40 G, -3 for Bailey-Barzal-Eberle
37.0 = 50 G, -39 for Olofsson-Eichel-Reinhart (using Sheary last year, Oloffson has no ES goals anyway)
34.7 = 45 G, -34 for Laine-Little-Ehlers line
32.0 = 41 G, -27 for Silfverberg-Rakell-Henrique
25.0 = 39 G, -42 for Keller-Stepan-Kessel (using Kessel's PIT stats)
The Habs line in 2018-19 was 7th out of 33 lines in ES goals scored/plus-minus. They were 13th in goals scored alone, and 5th in plus-minus.
This is better than you probably thought but not as good as anyone who thought they were top 6.
Furthermore, they played in front of league average goaltending in terms of save percentage over 82 games, and a worse-than-average defence.
Conclusion: Danault-Gallagher-Tatar were in fact one of the better ES lines in the league, 7th out of 124 lines. But they are not elite because they don't dominate on the PP. Another thing is their defence this year seems not as good as last year, from my eye test. But you have to hand it to them for 2018-19, they were really, really good at 5-on-5.