I'd like to nominate Drowzee.
Drowzee might seem like an inferior Munchlax in terms of walling, but Drowzee does have the resistance to Fighting, and can also make for a decent Sleep absorber if you don't wish to use Mankey. Drowzee also gets support options in Dual Screens, Trick and even Wish. Drowzee can also use niche moves like Guard Swap, Trick and Role Play. Drowzee can also spread status with Hypnosis and Thunder Wave. It's offenses may be lacking, but I think Drowzee has tremendous unexplored potential.