I noticed that
Seismitoad hasn't been mentioned yet in this topic. While it's quite unpopular, I believe it has a solid niche in today's metagame as a Swift Swimmer on rain teams.
The most important thing Seismitoad brings to rain is a Thunder Wave immunity and a very solid offensive Thundurus switchin. Thundurus is a scary Pokémon for rain since Swift Swimmers rely on their speed to sweep and Prankster Thunder Wave shuts this down: it's the great equalizer, enabling offensive teams to stand their ground against a team archetype that would otherwise outpace and overpower them. Toad is respectably bulky on both sides of the spectrum and is only weak to the (very rare) Grass Knot; in the rain, Toad outspeeds and ohkos Thundurus with Hydro Pump, even w/o rocks.
Toad isn't just an electric resist on a stick, of course. His mixed bulk is respectable, similar to Kingdra's, and like Kingdra Seismitoad has no exploitable weaknesses to priority moves. In the rain, Seismitoad has enough speed to outpace every metagame-relevant threat, including Deoxys-S (though sadly he does need to run a Timid nature to reach this landmark), which is obviously a big deal for a sweeper that relies on high speed instead of priority. Toad's base 85 special attack seems low relative to other Swift Swimmers, but it's really not a huge deal- Life Orb, rain boosted STAB Hydro Pumps are incredibly powerful, even coming from a low base. What's more, Seismitoad has a respectable special movepool that includes great moves like Sludge Wave and Focus Blast. Hydro Pump + Sludge Wave 2hkos Azumarill, while Focus Blast cleanly 2hkos Ferrothorn- these two Pokémon are obviously incredibly important targets for rain teams, as their bulk and resistances can trouble other Swift Swimmers. A carefully played Toad can weaken or even outright KO these threats, making it easier for other Swift Swimmers to clean up.
Seismitoad isn't an amazing Pokémon and he should never be the first choice Swift Swimmer, but I feel he's a useful part of the rain offence toolkit. Thundurus-I is everywhere, and Seismitoad is the only Swift Swimmer immune to his Prankster T-Waves- the most common method of speed control in OU. While he has no standout stats, everything he has is good enough to allow him to function in OU, and as beefy electric types such as Raikou become increasingly common, Toad's niche is only going to become more useful.
The only set I'd be advocating is a Life Orb/Swift Swim sweeper. I don't feel as though Seismitoad has any real niche in OU this gen outside of this role, but I feel this niche is worth writing about.
