Early Tank was surely strong. He came closer to tossing someone out of the cage than anyone else. This question is always hard to answer, though, and gets a bunch of different perspectives. Are we talking weight-lifting strength or functional MMA strength (however you define that)? And what about technique? A D-1 wrestler can make a stronger powerlifter look silly. Here's a quick list of some of the strongest HWs over the years, though:
Early Tank
Early Goodridge
Mark Kerr
Tom Erickson
Brock Lesnar
The "Ubereem" version of Overeem (not the current USADA one)
Pudz (the overall strongest based on his pre-MMA accomplishments...doesn't necessarily show it in the cage like someone with better technique, though)
Maybe Ngannou
Sapp is way up there too...if we're counting super HWs. There's no way he could ever make 265, though.