Bed of Chaos and Anor Londo archers for cheap kills.
Four kings and Ornstein/Smough otherwise.
4kings is difficult if your dps is low and O&S is easily the hardest boss in the game solo.
Another notable mention is the skele dogs in ToG. Especially on NG+ and beyond.
Funny, on my very 1st playthrough I solo'd O&S on my 2nd try, no problems (BSS +10)...but on my next NG with STR build (BK Halberd +5) I kept failing and had to summon Solaire, and even then it took me, like, 7-8 runs. Blah.
-O&S, these guys are easy peasy...If you are with at least another phantom or killed one already. By the time i took one down i was out of flasks (Dont have the rite of kindle yet, so thats an amazing 5 flasks for the entire game :) I could get lucky and get one to half health in the 2nd part of the battle, but i would simply run out of flasks... With host+phantom (one is enough)? Probably the easiest boss.
-Four Kings, they are easy, predictable and dont hit that hard...But if i cant take them down fast enough im screwed.
-Archers in Anor Londo, simply because archers in this game are cheap, since they dont lead theyre shots, they actually have homing arrows. When i noticed that i got really mad, because DS dificulty is all about trial and error and skill, not cheap s****.
Getting Blue Titanite Slabs as drops with 410 item discovery (as of patch 1.06 without DLC). I haven't gotten a single slab from first two butterflies in the Crystal Cave even after several dozen runs. Only chunks (and not even them more than half of the time). Navigating through the invisible walkway to get the slab from the corpse is much less of a hassle.