Ninetales and Politoed - B-
Outside of summoning weather, they both suck. However, that is still a good niche.
Cresselia - A+
Amazing bulk, great support movepool. Can easily set up Sun, Rain, Trick Room and Gravity, also has Lunar Dance to safely bring in a weather summoner if you're using one. Speaking of...
Tyranitar - A
Unlike Politoed and Ninetales, Tyranitar has utility outside of providing weather. It hits very hard, and can reliably set up Stealth Rock. It can also work on Trick Room and Sun teams as well
Hippowdon - A-
Better bulk than Tyranitar, and being the slowest weather summoner around helps if both get switched in on the same turn. Still hits decently hard thanks to 112 base Attack
Conkeldurr - A
As Megaqwer said, under Trick Room it's lethal. Give it a status orb (Flame Orb's usually the best option, as it's meant to sweep), it hits like a freight train. STAB Hammer Arm makes it even "faster", and between Hammer Arm, Knock Off and Facade it has perfect neutral coverage. STAB priority's always nice too
Whimsicott - A-
Priority Sunny Day and priority Taunt to disrupt opposing leads. It can also set up Trick Room, and abuse Misty and Grassy Terrain with effectively a priority STAB thanks to Nature Power.
Aerodactyl - B-
The fastest user of Stealth Rock and Taunt (bar its mega forme). It can also set up Sandstorm. Frail, but its great Speed does somewhat make up for that
Forretress - B+
Great bulk, can set up all entry hazards bar Sticky Web as well as remove them, and can even set up Sunny Day and Gravity. Can also use a slow Volt Switch / Explosion to bring in a weather summoner.
Bronzong - B+
Not as bulky as Cresselia, but has better defensive typing and unlike the celestial swan it can set up Stealth Rock and Sandstorm. It can abuse Trick Room itself reasonably well too.
Rampardos - C-
Scyther NO Swiping bought up Rampardos in
this article. While it could be devastating under Trick Room, honestly I think it's mostly outclassed by Conkeldurr and Rhyperior - they're both slower and bulkier, and while they don't hit as hard, neither is what you'd call weak.
Gengar - C-
Fast, frail Pokémon in general struggle in this meta, and Gengar has it really bad. Not only does it get murdered by Trick Room, most Swift Swim users demolish it too as does Sand Rush Excadrill. It doesn't even get priority, and can't use Focus Sash nearly as well as Alakazam.
Mega Swampert - A-
For Rain teams, Swampert is probably the best mega you could use. 150 base Attack is pretty damn high, and in the Rain Waterfall has the same power as a STAB Close Combat. Thanks to Swift Swim it outruns everything without a Choice Scarf even with an Adamant nature. It should only be used in Rain though.