Mamoswine

Mamoswine is a Pokemon with many great traits, and many serious flaws; starting with the flaws, it has 5 weaknesses: Fire, Water, Grass, Steel, and Fighting, most of which are easily exploitable. This, combined with one useless resistance to Poison, one immunity to Electric that is difficult to take advantage of, and subpar Speed and defenses, greatly hamper Mamoswine's potential.

However, it's in the lead position, where these flaws are less significant, that its traits truly stand out. Stealth Rock, a fairly powerful STAB Earthquake, STAB priority that destroys Shaymin-S, a demon to most suicide leads, and the ability to retain its Focus Sash in both sandstorm and hail, combine together to make Mamoswine a surprisingly effective suicide lead.