Board Thread:General Discussion v2.0/@comment-26347196-20151109173852/@comment-5956954-20151114231142

well, technicaly, sans is the hardest because unlike the other final bosses, nothing help you throughout the fight, like omega floweys healing and checkpoints, and asriel dreemurs because you can't die and you have checkpoints.

if the perks on both the asriel dreemur and omega flowey battle were removed, they both will be harder than sans with asriels third forms rapid burst bullets that cover the screen, and omega flowey for being himself.

so, in this case, omega flowey is the hardest for being neigh near impossible due to the many amount of hits that are hard to avoid, and asriel will be a little harder.

but due to technicality, the vote goes to sans.