Dragrath
Duke Fishron
In normal mode I would agree with you, but in expert mode 0_oDuke Fishron has always been easier for me, once I got the hang of him. I wouldn't say he's a cake walk, but with practice, he's a relatively easy boss, and one that's actually fairly easy to do a "no damage" run on.
There seem to be some people who can never figure him our or beat him now matter how many times they try, and that's why that stands out to me.
Uh yeah I will not touch this guy until I have the lunar tier equipment... That phase 3...
The Destroyer has always been the easiest mech thanks to the probes sometimes dropping a heart. in fact On expert mode I can only beat the twins if the destroyer is also present... (though this requires an arena to deflect his laser spam which will murder you)
The Cultist is also pretty easy once you get the pattern down, still a fun fight other than the lack of loot...