If this is an intellectual exercise, OK. But if this is going to influence a purchase, my advice would be to don't be tempted to buy old iMacs. Apple really only gives them about 7 years... so even if you buy 2020, that's 2 years on the clock now. The Intels are the most upgradable, so that's 2020 and older. Find the one that has flexibility about both RAM & SSD instead of either being soldered.
But again, 2020 is in the 2-years (to go) window, 2019 is in the final year, 2018 is computing dead, etc. IF you want Apple updates without leaning on the OCLP hack. The hack seems to make very old Macs run new macOS just fine but you are still depending on a hack layer to pull that off... which seems to offer easy imagination about security risks.
If it has to be iMac, Silicon is the way to go (but ZERO upgradability inside). Else, recreate an iMac-like setup by choosing a great monitor and maybe sticking a Mac Mini Base or Pro behind it. It too is not upgradable after you own it, but then you replace the whole puck instead of the entire setup as is the case with Silicon iMac.
If you OP are asking as a potential buyer, Caveat emptor! It's over for the Intels. It's complete lockdown for the Silicons.