Yeah for me I find Warbreaker most underrated. I find a decent amount of fans saying its dull and unimportant, while criticizing the magic system.
Elantris on the other hand is hyped even though it is quite dull next to most of Brandon's stories. Gary Stu Raoden, Mary Sue Sarene, Hrathen was the only strong character. Boring politics, boring magic system, very basic shardworld. It gets all this hype, to be fair Brandon wrote this when he was less experienced, and I'm sure the elantris sequel will be much better due to his improvements in writing. I don't get why Elantris is so popular, despite almost every Cosmere book being better than it.