Radahn is pretty damn tough in the grand scheme of souls games, but I don't think he is the hardest boss ever, unless you are either making the fight hard or are trying to pure caster that shit. Melenia demands more perfect execution and/or specific builds to solo, so she is at the minimum comparable. Midir is a total rape fest unless you build around parrying him (which parries were much harder in DS3) and even then you have to have some very specific builds to take him on, plus the fight is cheese proof. The two cats fight iin DS2 was rage induing, but thats probably more down to the run back than the actual fight. The fight against the three summons in the DS2 DLC was also fucking brutal.
Radahn's biggest challenge always has been seeing what the fuck he is doing with all the hopping around and diarrhea Christmas lights bullshit. By that point, the spammy bullshit is something you are used to anyhow. But if you want to do anything other than counterpoke with a bleedspear and shield, being able to read his windups is essential.
Midir and Gael are the only two bosses left in the entire Soulsborne universe that I haven't killed. Didn't really try Gael, DID try Midir and couldn't do it. By that point in Souls 3 I was pretty tuckered out with the game and trying over and over wasn't that appealing anymore. Can also more or less add Nameless King to make it a trio, because even though I technically downed NK, I died enough times to resort to summoning other people and they just ran up and destroyed him, I probably did 10% of the damage. Midir on the other hand I couldn't get down no matter what, and my build was poorly-suited for it.
So basically I really need to do a second run of Souls 3, but it's kind of far down the list now when I have unfinished stuff like Lies of P Overture, Wuchang, and Nioh 1.
Oh yeah, stats at the end of SOTE: Scadu level 16, Revered level 7, character level 255.
Used 2x +25 Blood Bandit's Curved Sword, Dragoncrest Greatshield Talisman (defense), Lord of Blood's Exultation, Millicent's Prosthesis, Rotten Winged Sword Talisman.
I didn't get that many chances to spam attacks on him, so I probably would have been better off switching the last two talismans out for the jumping attack talisman and the Golden Braid for holy resistance. Turned out holy damage was way more of a threat than anything else on the fight.
Now that I've got all of Shadow of the Erdtree beaten, I'd rank the boss difficulty like this (Malenia and Elden Beast included as a benchmark) -
Tier 1: Radahn
Tier 2: Bayle / Malenia
Tier 3: Rellana / Commander Gaius / Messmer
Tier 3 and a half: Those stupid Furnace Golem assholes
Tier 4: Metyr / Midra / Elden Beast
Tier 5: Scadutree Avatar / Putrescent Knight / Dancing Lion
Tier 6: Everything else in the DLC
That's just me though, everybody's gonna have different fights that end up being easier or more difficult for them for various reasons.
Bayle was all about getting through the initial 20 seconds and stabilizing the fight.
I found that a lot of the DLC bosses were all about surviving long enough to stabilize the fight. Lost count of all the times I died barely out of the doorway of boss fights. Only time I remember that happening in the base game was Radagon a couple times since he has that dashing decapitation swing that can catch you.