Any bosses on this list marked with * are optional, but I'll explain each one. A few spoilers ahead. Nothing too major in my opinion.
The Capra Demon, and by extension the Gaping Dragon, are skippable because you can enter Blighttown through the exit by utilizing the Valley of Drakes. I recommend learning the Blighttown Drop to make it easier for yourself, and fighting the Capra Demon for the ember in the Depths and Kirk's equipment if you want it.
Ceaseless Discharge can *technically* be considered optional because there's a bug to skip him, but you should just fight him. The skip is VERY difficult and I'm yet to even pull it off myself. He's easy to cheese and you'll miss out on a really good armor set.
You can just walk past Priscilla. She doesn't care unless you whack her.
Gwyndolin is out of the way and will break the darkmoon covenant if you fight him. Just leave him be unless you REALLY need something behind him, because I think it'll piss off the fire keeper too.
Demon Firesage and the Centipede Demon can be skipped if you join the Servants of Chaos and rank up all the way by offering 30 humanities to Quelaag's Sister. This is also necessary for saving Solaire for the final boss.
The Moonlight Butterfly is optional, simply for being slightly out of the way, is another boss you should fight anyways. You get an ember and a
key to the Lower Burg and by extension the Depths and Griggs,and a bunch of stuff.
Taurus Demon can be skipped by using the Master Key to loop around through the Darkroot Basin. Just fight him, he's easy enough and lets you talk to Solaire for the soapstone and his quest.
Stray Demon is back in the Tutorial Asylum and gives you a titanite Slab upon defeat. You should fight him for the slab alone, but there are other secrets back in the Asylum, like the Doll to take you to the Painted World or the ring that helps you walk through deep water or mud normally.