Flame lurker was my favourite boss fight. I literally jumped up from my couch and fist pumped when I beat him on melee combat. Best gaming feeling and moment ever!! Demon's Souls made you care sooooo much about your character's life. Your character's life meant so much to you because it took you back so far. It almost felt like your real life was interlocked with the character's. Other souls games are fun, but I care less about my character's life. Bloodborne is the only other Souls-ish game which is as satisfying as the origin Demon's Souls.
Pretty much everyone who beat Flame Lurker on Melee combat had the same exact feeling, you can take a look at youtube and see how people reacted after beating that boss. Whilst Flame Lurker was the hardest IMO, there were other bosses and instances where I had similar feeling, especially with Maneater, except it was more infuriating with Maneater especially when they push you and make you fall of the ledge.