Since MediaTek and Broadcom are completely out of the equation, which are the SoC brands that better support by far, the open source community?
Like if they released a new device tomorrow, we could be 100% sure that the opensource drivers will be in their page ready to work with.
From my research I would personally say Qualcomm Atheros, but maybe I'm wrong or there is just a much better manufacturer.
Why? When I read the title, MediaTek was what immediately came to mind as an answer. I'm curious why it's out of the question for you?
True, I see that MediaTek has a good support for OpenWRT. Probably I was biased with those Android MTK devices that have one of the most terrible support for custom ROM. But seems that in the router spectrum MediaTek is working somewhat well releasing info, which surprises me a lot because everytime I've had the need to buy or recommend an smartphone, Mediatek in any of its variants (from Helio to the regular MTK) is a no.
But now I will not oversee MTK overall for routers. The database is nowadays almost as gigantic as Atheros.
So we could say that probably Atheros and MTK are currently the leaders in this OS aspect?
Mediatek first, Qualcomm second and a long way back.
Only some Qualcomm socs get support and from watching the development threads it’s getting more difficult with each generation
Very interesting to find consensus on MediaTek I would have never said such.
Apart from these brands, any other honorable mention?
The most honourable mention for all other alternatives (apart from x86_64, the SBC like devices (RPi4, rockchip) and some wired-only things like qoriq or octeon/cavium and wired-only mvebu) would be, [SILENCE].
Silence?
I assume with x86_64, you mean Intel iwlwifi?
Linux Plumbers Conference September 2022: All types of wireless in Linux are terrible and why
the vendors should feel bad
Since then, i know that finally some 5GHz capable Realtek USB devices have made their way into the linux kernel (As far as I am aware, this effort has been led by enthusiasts and not by Realtek itself 
). Things are ever changing, so let's not get hung up on these things.
Yes, some manifacturers definitely could improve their OpenSource support in the wifi realm by properly and more often upstreaming driver/firmware into the Linux kernel...
You also can find out easily who leads in terms of OpenSource support to Linux/OpenWRT by looking at the list of 802.11ax (wifi 6) supported devices, as ax is the most recent wifi standard (soon to be replaced by wifi 7):
It's mostly Mediatek and a few Qualcomm devices. Of course, this list does not take into account the client side e.g. Intel ax200 devices etcetera.
Edit: Be aware: this list says nothing about the QUALITY of drivers/firmware 
Not mostly, only Qualcomm and Mediatek devices
Definitely I always put a bet on Atheros since WR1043ND which was my first go-to device.
Then it's true that I started finding value in first Mediatek devices that seems to "open" the code like D-Link DIR-882 but truly I never knew that Mediatek was open in this regard. I always though it was just a coincidence so I never purchased again a Mediatek device and sticked to Atheros/Qualcomm.
But knowing this, I find on MediaTek a TON of really interesting devices I will be looking forward now for new projects, like the 2021s ASUS RT-AX53U I would have completely disregarded one week ago because if it's MediaTek chipset.
Also, it's intriguing the hatred that most DD-WRT maintainers have for almost all MediaTek in the forums. I am uncertain if its justified or not.
Yes, this is a different regard, very difficult to dig in, just reviewing each device individually. I can see in each device a ton of comments on the bright and the dark side of each. Here we could enter a different discussion that, I think, is more common on this subforum (recommended hardware overall). But still, personally I like to do a first screening based on brands and chipsets, before digging into specific devices one by one. Otherwise each research every year could take ages to accomplish a conclusion. So I will be including MediaTek from now on.
FWIW, I've been asking the same question myself, and recently decided to replace my old Linksys WRT mvebu router with one using a MediaTek chipset.
There are a couple of really nice and affordable routers running Qualcomm chipsets right now, but some of the comments about the difficulty maintaining and working with the athxx drivers made me decide to support MediaTek instead.
This old talk by Felix Fietkau about linux wireless driver history, internal architecture, and manufacturer support was also a big factor in making me want to support MediaTek instead of Qualcomm with my money.
