Dark Souls I
Favorite: Ornstein and Smough
The two are just amazing. They have great lore behind them, are fought in a great location, it's a fun and hard battle, with both of them using very different techniques. And the music! O&S for life!
Hardest: Manus
Well, this guy.... yeah, he's just everywhere xD I really struggle with him on each playthrough. Manus punishes aggression, but he also punishes defensive approaches. I've no idea how to properly approach this guy, and each battle I just... fake it till I make it, I guess. Yeah...
I really had a hard time picking a favorite boss in the first game. I love quick, aggressive, action-packed battles. I considered Artorias, Sif and the Four Kings.
Dark Souls II
Favorite: Smelter Demon
As I said above, I love quick and aggressive battles, and this guy gives me just that. It's not easy to dodge this guy, as he swings his sword at different intervals (sometimes he'll wait longer before swinging, other times he won't). And then he lights himself and the sword on fire. He has a huge pool of health, and is overall a great, long battle.
Hardest: Throne Watcher & Defender
I don't know... I really had a tough time dealing with these two. Battling them requires both holding back and being aggressive, depending on the moment, and it was really hard for me to kinda bait one of them in order to attack. Each of mine attacks ended up with me taking damage, even dying often. The first time I defeated them was with both NPC summons (Bernhard and the other), and even then I had a hard time!
It's worth mentioning that I didn't play through the DSII DLCs, and therefore my choices here aren't optimal.
Dark Souls III
Favorite: The Abyss Watchers
Holy hell, this boss is amazing! The watchers have a perfect design, perfect battle style and a great lore! It's inexplicably fun to duke it out with them. And the second phase is even better. Oh, the beautiful flames that come off his sword. Yeah, I love the Abyss Watchers.
Hardest: The Nameless King
I'll just say one thing: Screw the camera in this fight! The first phase is much harder than the second one IMO, just because of the camera.
Well, Dark Souls III had a lot of great bosses. I considered Champion Gundyr or the Dragonslayer Armour as my favorite, too.
Bloodborne
Favorite: Logarius
Hey, it's Santa! Or not... this battle is just fun. Hard, but definitely not the hardest in the game. Logarius is just a boss with decent lore, great design and a really great fighting area. He forces you to be aggressive, to play it quickly. Hold back, and he punishes with his magic. Just approach and be vary of his scythe.
Hardest: Rom, the vacuous spider
Screw this potato, and all of his potato-head babies! The first half of the battle isn't so hard, but then he starts throwing giant ice crystals at you if you're far away, or does one out of two AoE attacks if you're near! All while you have to either find a path through the small spiders, or kill them all. This boss really punishes my playing style...
Gee, Bloodborne had a lot of great bosses. It was hard picking my favorite. I considered Gehrman, Darkbeast Paarl, Ebrietas and Gascoigne.