Well, my first playthrough I had a lot of trouble with S&O (I hadn't ever upgraded my weapons...) but after trying so many times to beat them they seem pretty easy now. On a non SL1 run, I don't really think any of the bosses are very hard once you know what to do. Really, I find the titanite demon in that room in Anor Londo more intimidating than any of the original bosses. Counting the DLC bosses though, I did find a decent challenge in Manus.
Easiest boss is fairly easy to pick; I'd go with Ceaseless Discharge since you can kill him without a single attack. Runner-up: Iron Golem, it's too easy to knock him off the bridge Runner-runner-up: Gwyn, equip a target shield/buckler and this is a ridiculously easy fight, very underwhelming for a final boss.