Smeargle

Smeargle learns almost every move, so it is dangerous through its unpredictability. In particular, there is no way to know what a lead Smeargle is going to do for sure. It has access to Spore, a reliable sleeping move that no other viable Pokemon can use, and with access to Baton Pass and Spikes alongside Spore, it can do a solid job of supporting an offensive team. However, the fact that Smeargle is slower than many common leads and can't take many attacks restricts its viability. In addition, its user has to make key predictions around the timing of its use of moves like Spore, and choosing the wrong move or the wrong turn to use a specific move will typically result in Smeargle becoming fodder. Straying from the sets outlined in this analysis may be a wise choice, because while they are effective, people will expect them, and one of Smeargle's most valuable traits is its unpredictability.