Guys like Samart Payakaroon, Khaosai Galaxy and others who have competed and succeeded both in elite pro boxing and elite pro kickboxing/MT should be expected have a strong advantage with hands, as well as every other area of boxing.
As long as they can effectively implement both against their opponent, they are generally at the advantage in the fight as well.
I could imagine Samart and Khaosai beasting top kickboxing divisions such as Glory and K-1. Smart's defensive abilities were beyond those of anyone I can think of in modern kickboxing/MT today - Petrosyan included. Slick movement and speedy reactions made him a difficult target to box against. He also countered extremely well. This also was reflected in Muay Thai, as he often rendered his opponents' kicks useless with split-second teeps. Petro was pretty good at stopping/avoiding kicks, but I still give Samart the reactive edge. Prime Samart would be a challenging to extremely difficult matchup for a striker of any style.
There are exceptions; some of the best fighters in kickboxing today and past include those who have received extensive boxing training complementary to their kickboxing. I'd think of Giorgio Petrosyan, Tyrone Spong, Rico Verhoeven, Nieky Holzken, Jomthong Chuttawana to name a few. Masato was a skilled amateur boxer before he transitioned to a kickboxing career. He ended up knocking out Virgil Kalakoda, who honestly wasn't my favourite reference for a boxer in K-1, but had more than enough boxing experience to make him dangerous. Zack Mwekassa wasn't a significant pro boxer by any means, but he was good enough to make it to the finals in Glory. Most of these
Traditional boxing is reintegrating itself again into mainstream kickboxing again, somewhat like the American kickboxing days of the old, and even the early K-1 days. Just that champs will always be champs, and being amazing at multiple combat sports really brings hell loose.
TLDR; I think all time I'd say any of a smaller number of fighters who have won prestigious world titles in both boxing and Muay Thai. Even prime Somrak Kamsing would badly outbox most of the kickboxers today.
