OK, update coming
As per your suggestions, Politoed and Ninetales will be moved B+
Thundurus is going to A+ - can set and abuse rain, also has priority Taunt and Thunder Wave and a great movepool
Volcanion is going to B+ - good on rain, sun and grassy terrain
Chandelure is going to B - hits like a freight train in Sun, and can also counter set Trick Room
The issue with Mesprit is that between Mew and Cresselia it has virtually no niche. It is something of a hybrid between the two, but Mew and Cress both have reliable recovery so they can set up more effects more easily.
Some other Pokémon to discuss.
Mega Venusaur B/B+/A- - one of the best Grassy Terrain setters, and a good check to offensive Rain teams as well
Rhyperior - B / B+ - A great Pokémon to use in Trick Room due to its great physical bulk and Solid Rock means it doesn't fear priority that isn't Aqua Jet. Swords Dance coupled with STAB EdgeQuake makes it tough to stop in Trick Room, although it needs Trick Room to shine
Talonflame A- / B+ - Loves Sun teams, and while fast frail Pokémon normally struggle in this meta thanks to Gale Wings Brave Bird Talonflame isn't too bothered by Trick Room.
Serperior A- / B+ - One of the better Grassy Terrain setters, it also has Taunt to stop other slower leads setting up. Can set up Sunny Day as well, and still hits very hard after a Leaf Storm
Froslass B - A great Spikes lead due to its good Speed, Taunt, Thunder Wave and Cursed Body. It can also set up Rain
Also thanks to the people who voted for Lockdown, shame it didn't win but oh well.