Lot of ways you can go with this. I assume you mean underutilized, not just someone who isn't used often like Undertaker or Brock Lesnar.
So by my definition I'm listing the people I consider to have the most potential in the three upper-tier promotions who aren't booked to best take advantage of their potential.
Off the top of my head I'd go with:
10. Alex Riley
9. Tensai
8. Natalia
7. MsChif
6. R.D. Evans
5. Velvet Sky
4. Wade Barrett
3. Damien Sandow
2. Dolph Ziggler
1. Miz