Seconding this. Sen''s is a simple matter to finish, but an awful maze the first time yoou run through it.
I like Anor Londo a lot, though that actually feels shorter in some ways because it's broken into bite-size chunks.
Spoiler: Anor Londo
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1) Kill Gargoyle
2) Chapel rafters, 3/4 enemies
3) Gargoyle 2
4) Cathedral exterior with a grand total of 7 enemies you need to care about, mainly challenging because of the Dragonslayer Bow knights
5) Cathedral interior (3 knights at most using a direct path)
6) Boss hall and Biggie and Smalls (Only O&S can't be easily dodged around
So the whole area only requires you to kill 16 enemies and the boss.