Torterra for In-game, Shuckle for post-game
For in-game playthrough, Shuckle is terrible. Complete garbage, with no offensive threats due to horrendous speed and offensive stats. Torterra is leagues ahead in this category.
For Battle Frontier, most of the battles are shorter, and Shuckle and Torterra are both good but a Rest/ Toxic / Protect / Substitute Shuckle would be very useful tactic to use against the AI (barring Poison or Steel types and taunt users).
For online battles, Shuckle is again quite useful as a lead, for setting up Stealth Rock and trapping your opponent with Sand Tomb and wearing them down with Toxic. However, while Torterra is a decent pick as well, it is generally too slow and 4x weak to Ice Beam (a super common coverage move) to be a huge threat.