The Hobbit and Lord of the Rings, and whispers The Percy Jackson series... Altho maybe not so much boring as I just didn't enjoy it, even though the concept is interesting.
A book series I found written incredibly dull but somehow still enjoyed was the Howl Moving Castle series XD I really love Howl's character (he even influenced my MC a bit, lol) but the writing style to me is so boring. The second book, Castle in the Air, is especially dull up until the ending. House of Many Ways also has a stronger ending than beginning. I've still read them multiple times anyway though, lol.