So I did end up wrapping Dark Souls 3 a few weeks back, including capping it off with first trying Midir and Gael. And of the two, I gotta say the one whose difficulty I found somewhat overhyped was Midir. Sure it's an endurance fight with a galaxy sized HP bar, but none of his attacks were very difficult to dodge given they were highly telegraphed and had very generous iframe windows. The first time not knowing what to expect at first I ended up beating him with only using 7 Estus, and after fighting him a few more times by letting people summon me for him, I honestly think I could probably beat him hitless on a solo run. Although I'd say phantoms make him slightly harder because if everyone doesn't stay squared up, his random turns to face other players can definitely throw timing off... if not invoke the annoying tail whip.
I mean I can see why he would give people trouble if they didn't get the hint to stay squared up (which clicked pretty quickly when I realized the head took more damage and he stop spamming the downward fire breath if you're not under him), but considering how many people/articles/etc seem to hail him as one of the hardest bosses in the series... I dunno if I found him to live up to the hype. It felt very scripted when I realized how easily you could bait his easy to dodge melee attacks by staying close to the head, and even most of his fire wasn't too difficult to dodge, again assuming you're not standing directly under him.
Definitely a fun fight though, and one I enjoyed doing enough to let random people summon me to fight him again.
DS3 was definitely my favorite of the three and my final rankings would be: DS1 - 7.5/10, DS2 - 6.5/10, DS3 - 9/10. The faster speed and more grandiose boss battles of DS3 sealed the deal as an easy favorite, despite it being the most linear of the three. But linearity isn't in and of itself a bad thing in my book as long as it's fun to play.
Faron Keep and Irythill Dungeon notwithstanding. At least the former had the incredible Abyss Watchers to make it all worthwhile.