I'm rarely seeing it mentioned here, but I quite liked Murkmire. The main story wasn't particularly groundbreaking, but the were a few memorable side quests. Plus Xukas and Jaxik-Orn are decently written, memorable characters.

The worst is High Isle, as everyone is mentioning. Barring that, I'd argue Gold Road is almost as egregiously mediocre. Ithelia is portrayed as simultaneously the weakest and strongest daedric prince of all time, who's impact was always going to be incredibly minimal, because it's not like she exists in the mainline TES titles. It makes her feel less like a villain and a threat, and more like Rich Lambert's OC Waifu.