The answer is actually BJ Penn and if you disagree, you are objectively wrong.
He was inarguably the number 1 LW at one point and when he beat Hughes, he was inarguably the number 1 WW.
Jones didn't beat the reigning, lineal HW champ. Randy was legit HW champ but was never the proven 'man' at LHW because top talent was split with Pride - as proven when Rampage defeated Chuck (who beat Randy). Hendo lost both of his UFC 'unification' fights at LHW and MW proving he wasn't number 1.
GSP cherry picked a one-eyed man who nobody believes was actually the best MW and then ducked Whittaker. Alex is only a semi-legit champ, as the UFC has navigated him away from grapplers and he won the LHW belt that was vacant. The LHW belt is a joke, with multiple champs vacating.
Shillmier has always lived in Jones' shadow. Cejudo lost the DJ fight and then beat a random guy for the vacant title who would then go on an epic losing streak. He only defended a washed up old man in Cruz who had no business getting a title shot in the first place.
The other guys fight in bum leagues and aren't worth mentioning.
Strange to think old mate Tippy Toes was the most legit double champ. But life is strange. Only guy to ever be clear cut the best in two divisions.