Rusev
It's difficult to really decide the most underutilized Superstars in WWE, as the case could be made for so many. Rusev is definitely on that list. He certainly requires more TV time. There are so many things for Rusev to do on Raw or SmackDown Live. He is great as the Bulgarian Brute, beating up Superstars to prove he can. He is a fantastic baby-face too. Rusev Day! Rusev Day! Those chants were so over with the WWE audience.
He could be used well in the upper mid-card. He does not need a title to have a good story with other Superstars. Maybe up and coming stars could challenge him, as sort of a gatekeeper type scenario to the main roster. Perhaps he goes on a baby-face run for the Intercontinental Championship. Maybe he does a series of vignettes showing his funny side or his relentless workout routines. Perhaps he coaches the next "super athlete." The possibilities are endless. Remember, if we get more of Rusev on TV, we also get more of Lana on TV.