If we are measuring by sheer difficulty for me living failures in Bloodborne was harder than anything, because that's the only boss that actually got me stuck (it took months of off and on playing where I'd try 3-5 times and give up for a while only to come back) or made me actually grind and meta game in order to get the edge necessary to beat them. The worst part is that by the time I finally beat them, I think I'd helped over a hundred other people beat them to, but while I'd be summoned within 7-20 seconds of ringing my coop bell, whenever I tried to summon there would be nothing but silence, run after run, leaving me to solo him every damn try! I hate those abominations so much, I doubt I'll ever play the old hunters dlc again for the simple fact that I know they'll be there down the line. Definitely not a favorite, but it's my most memorable one just the same. Most people usually attribute these frustrations to Orphan of Kos, but somehow I managed to solo-kill both Maria and that poor wretch on my first try once I finally got past living failures .
As for bosses I enjoyed.
Have to agree with awesomecool overall regarding Artorias/abyss knights though OP does make a valid point. Artorias was a much better fight overall, but abyss watchers had a better ost, and the half-fight revive added a lot of tension at first. But then you realize that his hp bar still chips rapidly for a boss and his flaming sword and lack of allies/enemies actually makes him even more predictable than the first half of the fight. Artorias was a threat, in the end while I enjoyed the atmosphere of the fight Abyss watchers was less of a challenge than the two black knights you have to fight right before his chamber, making him memorable for his awesome cutscenes but otherwise disappointing as a boss.
Gascoine? (second boss of bloodborne) Obviously in terms of actual difficulty he'd be a joke compared to later bosses like Maria, Gerham or even some of the later npc miniboss fights, but the old priest was memorable to me and probably nearly anyone else who opted to solo him because he was the first that really forced you to adapt to bloodborne's quickend battle pacing or die. Up until that point I was a very defensive player in bloodborne, and would miss opportunities because I didn't know suicidal offense was always the best defense. In my struggle to beat the old priest he taught me the gun-stun thingy (by doing it to me), then accidentally the monster arm impaling thing and just to add insult to injury I misinterpreted his paleness and fangs in the opening cutscene to mean he was a vampire, so when he first turned into a werewolf it threw me off just long enough for him to rip me to shreds. Each loss felt like a learning experience, then had me come back and use what he taught me to finally kill him (like a first souls game again). That thrill was never really relived after that, by the time I beat him I had a pretty solid grasp of the mechanics and spent my time refining them so while he was easily the easiest humanoid enemy in the game, his fight was also the most memorable to me because by the time I fought those harder bosses I had things down pat and they fell much more easily to me than Gascoine did, not even Gerham could invoke the same feeling of inferiority that Gascoine first did, though every scrap of lore on him set him up for just that.
Sinh the dragon: Come on, no one liked Sinh so far? After all the dragon boss fails from Dragon God to the unbelievably easy Guardian dragon and no one else enjoyed the thrill of finally fighting a dragon that was actually challenging for once as well as epic as it flew around the vast underground arena, poisoned you and then landed to eat you? Hands down best dragon, to bad he exists in the game that has so few other memorable bosses and suffers greatly by opting clusterf*** over substance in regards to non-boss encounters.
