BB(2,8)
The 2-state 8-symbol Busy Beaver problem, BB(2,8), is unsolved and unexplored. With Cryptids like Hydra in the preceding domain BB(2,5), we know that we must solve a Collatz-like problem in order to solve BB(2,8). The current BB(2,8) champion is simply inherited from BB(2,6) as BB(2,8) has not yet been explored.
It is known from the enumeration of BB(2,7) that BB(2,8) will be hard to enumerate.