Tough one. I've played and platinum'd all 4 and it's a bit dependant on various factors.
If you don't cheese, Maneater is an absolute ball ache. The same could be said however for bosses like Amygdala in bloodborne (easy with bowblade and the holy blade, tough without), the poison dragon in the Ds2 dlc (a piece of piss with a good bow) and the four kings in DS (if you have a high dps, defence isn't an issue). Same again with play style (for example, a player good at dodging will have no problem whatsoever with 95% of DS2's bosses, see my YouTube channel "PheoDex" if you don't believe me, and a lot of the bosses in bloodborne are very easy if you are particularly good at setting up visceral attacks) and build (the snake queen lady in DS2 is a joke to a pyromancer).
As a player who favours Dex/skill based builds, is reasonable but not perfect at parry/visceral attacks, and always preferrs dodging to shields, I'd say the bosses that personally gave me the best challenge were the Maneater (DeS), Old King Allant (DeS), Ornstein and Smough (DS), Knight Artorais (DS DLC), Kalameet (DS DLC), Sinh the slumbering dragon (DS2 DLC) and Ludwig the Accursed (Bloodborne DLC). Going by amount that makes DS the most challenging to me. Honourable mentions however go the the bloody crow of cainhurst (Bloodborne) who is the ONLY enemy in any of the games that I have never beaten without cheesing or co-op (but is technically not a boss) and the bed of chaos (DS) who is just an asshole.
In terms of enemy placements and level design, finding out the way to go without a guide the first time I played, Dark Souls wins hands down due to its open world and the fact that the graveyard area is both immediately assessable and blocked halfway until you progress through the game (first time I spent WEEKS getting my ass kicked, making it to the area after the darkness by sheer luck and then getting blocked by the force field. After realising I had to go all the way back I promptly deleted the file and restarted). The others were more linear and explicit in the direction to progress.
Finally, in terms of the challenge due to co-op, DS2 wins hands down. The PVP community is still very active and I still get invaded regularly by very skilled players who provide a fun challenge. DeS and DS are pretty much dead (invasions/PVP was very hit and miss anyway) and Bloodborne has very few PVP moments due to its online multiplayer structure (no surprise invasions except in spacific areas).
This is based on the PS3 versions of Demons Souls (duh) and Dark Souls and the PS4 versions of Dark Souls 2 and Bloodborne (duh again).
Soooo....... Yeah. I'm done now.
TL;DR Dark Souls is the toughest bosswise and in terms of its overall design, Bloodborne has toughest NPC players (well, one, but it's a bloody hard one) and Dark Souls 2 has the most skilled and therefore most challenging PVP community.