Mugatu and his piano-key necktie.
...
But seriously, I think it might help to think of them in categories. What brands/designers/tailors contributed to certain schools of thought or looks? So American/Ivy look, Neapolitan look, English look, Couture/Fashion look etc. Additionally, there are retailers (could be called "brands") that were innovators in both what they carried and the aesthetic they drove into the market even if they were less-than-successful business models. It would also be worth looking at what is driven the Japanese's buying as catering to their tastes has definitely had a huge influence on the luxury goods industry.
Intentionally leaving out non tailored clothing type folks so as to focus scope.
American/Ivy Style:
Brooks Brothers
Ralph Lauren
J.Press
Gant (Guaranteed to bleed, anyone?)
Oxxford
Paul Stuart
Hickey Freeman/HSM (HMX corp)
Alden
Allen Edmonds
(include bootmakers here? Red Wing, Frye, even the likes of Lucchese etc? larger question of what menswear means here)
English Style:
Hardy Amies
Anderson & Sheppard
Huntsman
Poole
Boateng
Nutter/Sexton
Barbour
Gieves & Hawkes
Turnbull & Asser
Burberry
Aquascutum
Cleverley
Lobb
Crockett Jones
Peal
Drakes
Italian Style:
Rubinacci
Brioni
Kiton
Corneliani
Zegna
Gucci (Horsebit loafers anyone?)
Ferragamo (Italian? He's technically a hollywood guy)
Barbera
Attolini
Fashion Style:
Hermes
Yves St Laurent
Tom Ford (including his Gucci work here)
Thom Browne
Pierre Cardin
Giorgio Armani
Interesting stores/business model innovation or preservation:
Arnys
Sulka
Loro Piana (really taken the fabric marketing to the next level, made it about the fabric and less about the designer)
Cordings
Ede & Ravenscroft (Who else still sells all the stiff collars, etc?)
an HK Tailor like WW Chan who really work that travelling tailor bit/fast bespoke in HK.
I'm sure I'm leaving out everything important but mainly I think breaking this up into categories, then sorting out the top 50 based on inclusion of these key spheres of influence is probably the easiest way to go about it.