I have answered this in the past but you have to factor in giving the other GMs Brady for 20 years, the GOAT QB on discount deals. If you do that then I would put:
Polian
Loomis
Wolf
Beathard
Davis
If picking from current GMs, I think Ballard, Veach, Loomis, Colbert would have had tremendous success over 20 years with Brady as their QB.
Edit - Sorry I didn’t realize you weren’t the original poster I responded to, so I interpreted it wrong. Your stance is not that he’s mediocre but that he’s a bit overrated, which I still don’t agree with. But let’s use that for the premise of my post below.
That’s kind of an unfair argument when you consider Belichick was who found Brady and convinced him to play at a discount in the first place. Guys like Polian, Beathard, and Davis did their best work before free agency was a thing. Belichick never had that benefit, he had to work around free agency, a system designed to not have 20 years of success.
Loomis had the Saints in salary cap hell, which would’ve been worse if not for Brees restructuring his contract and retiring. Also much like the Patriots, the Saints had their own HOF QB in Brees, one who also gave them team-friendly deals ($10m in 2013, $11m in 2014, $13m in 2017, all less than what Brady was making those years). Veach has swung and missed on all his wide receiver free agency targets so far, overpaid for Frank Clark, and inherited Kelce and Hill from John Dorsey. Ballard hasn’t shown anything other than he can’t seem to keep his QB’s around. Colbert is fantastic, one of the best today, but even he’s had his fair share of stinkers in the draft in recent memory (Jarvis Jones, Jason Worilds, Artie Burns, etc.)
Even disregarding all that, that’s not a lot of names. Polian, Wolf, Beathard, and Davis are all Hall of Famers, and you only listed four active GM’s. If Belichick was mediocre at best, wouldn’t he be in the middle of the pack?
One last thing to consider: Belichick the HC succeeds because he’s always had Belichick the GM to buy the groceries for him to cook with, and likewise Belichick the GM benefits from Belichick’s coaching, an advantage none of the other GM’s you listed have. If we need to give the other GM’s Brady for this exercise, why stop there? We can give them Belichick the HC as well.
I’m just having a hard time believing the man responsible for drafting these guys is “mediocre at best”:
Tom Brady
Richard Seymour
Matt Light
Daniel Graham
Deion Branch
Ty Warren
Asante Samuel
Dan Koppen
Vince Wilfork
Logan Mankins
Matt Cassel
Stephen Gostkowski
Jerod Mayo
Matthew Slater
Patrick Chung
Sebastian Vollmer
Julian Edelman
Devin McCourty
Rob Gronkowski
Brandon Spikes
Nate Solder
Shane Vereen
Marcus Cannon
Chandler Jones
Donta’ Hightower
Jamie Collins
Logan Ryan
Duron Harmon
Jimmy Garoppolo
James White
Malcom Brown
Trey Flowers
Shaq Mason
Joe Thuney
Damien Harris
Jake Bailey
Michael Onwenu
He brought in undrafted guys who became big contributors like Stephen Neal, Malcolm Butler, JC Jackson, David Andrews, Jonathan Jones, Adam Butler, etc., or the trades for Corey Dillon, Randy Moss, Wes Welker. The Pats don’t even compete in 2001 if not for the roster overhaul he did that offseason (Vrabel, Antowain Smith, Phifer, Cox, Patten, etc.) Belichick was responsible for all that.
He’s not going to hit on everything. No GM does. But no GM can match his resume of success, especially in the free agency era, and not even Brady would have 6 rings with the same team if his GM was merely mediocre at his job. All we ever hear from Packers fans is how Aaron Rodgers would have more rings if he had a better team around him, even though Ted Thompson, a highly regarded GM of his time, was running that ship for most of Rodgers’ career. If Belichick is mediocre then so is everyone else.