I've yet to truly enjoy a poison area in a Souls game so far. I'd say DS3's is my favorite across all the games though because it's sort of an interesting set up. If they didn't have the sections that you had to slog through or had an item you could find that let you move at normal speed, I'd be much more okay with the level overall.
This is also why I'm really hoping From/Miyazaki does a Sci-Fi Souls game because I think it would make then have to create new fresh environments, instead of "Okay here's the forest level. Okay here's the poison level. Okay here's the lava level. Okay here's the tomb level with skeletons." My favorite levels have been the ones that have had something unique about them. Tower of Latria in Demon's, Sen's Fortress in DS, the wharf in DS2, Old Yharnam in Bloodborne. Can't say any of the generic forest levels have really stood out in any game, regardless of how well designe: the layout may be.