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In terms of woodshops / brands, I don't have access to DOA boards, but from what everyone says they have the flattest.
I have some PGI wood which definitely felt very flat overall in the 8 and 8.125, even in the wider 8.5 size boards, but I would still only consider that mellow.
Most others including BBS, PS Stix, Clutch, Control, are anywhere from middle ground, sometimes more mellow, sometimes steeper, but others like Pennswood, Dwindle, even some other China made boards were steeper than average overall.
Most others I have identified from the woodshop list feel on the steeper side to me too, that is kicks more than 20 degrees and deeper concave, the one exception being most Powell boards now, but they just feel weird to me, sort of like everything is a bit off any time I stand on one.
Some HLC boards I have feel very flat through the concave, but have more medium angle kicks, on the Plan B and Sk8 Mafia boards that came through my hands, in 8 and 8.25 sizes.
Others like Watson and Prime decks are only the old school shaped boards and although they are very mellow, I don't count them in the same way a normal popsicle board would be, given they are reissues or boards that are meant to be like the 80s or early 90s boards.
If those decks were reshaped into popsicles of 8 to 8.25 they would definitely be flat, as per the 1996 and 1998 boards I have, which seem like they were made from those similar molds, so that would be the closest to what people generally call flat boards now.