Nominating Omastar for B+
I think omastar should be in B+ with its other swift swimming companions. It's the best special wallbreaker under rain. Specs hydro OHKOs almost every neutrality and 2HKOs most resists too. A simple set of hydro pump / scald / ice beam / knock off is extremely hard to switch into and even the premier special wall chansey gets 2HKOed under rain after losing its eviolite to knock off. Besides being very powerful, it can just like kabutops, also check talonflame (even better than kabutops thanks to better bulk) which is annoying for rain teams. Another interesting option for omastar is shell smash. This set needs more support to function, as it requires all priority users gone and needs to not have taken too much prior damage. You could call it high risk high reward. Basically if under rain you let this thing get a shell smash up and you don't have any priority users, might as well click the red x, lol.
However, there are some downsides to omastar, mainly its slow speed. It's still decently fast under rain but it can still get outsped by some common choice scarf users such as landorus-t. Other swift swimmers can outspeed basically the entire meta under rain, and while omastar can outspeed a decent chunk of the meta, it still gets outsped by some faster choice scarf users or pokemon like aerodactyl.
Comparing omastar to other swift swimmers:
Omastar pros:
Insanely hard to switch into
Checks talonflame under rain
Shell smash is deadly if it gets one off safely
Cons:
Slower than other swift swimmers
Weak to priority
Kabutops pros:
Swords dance
Rapid Spin
Powerful
Checks talonflame
Cons:
Weak to priority
Frailest swift swimmer
Swampert pros:
Bulky
Electric immunity
Decent SR setter
Cons:
Takes 1 turn to mevo
Kingdra pros:
Powerful
Can go mixed making it hard to wall
Offers grass neutrality
Cons:
Attacks have nasty side effects (outrage, draco meteor)
If you ask me, I'd say omastar is probably around the level of the other swift swimmers so I think it deserves B+.
Replays:
Vs #1 player on ladder
http://replay.pokemonshowdown.com/oususpecttest-236934876
Vs Get this money
http://replay.pokemonshowdown.com/oususpecttest-236936158 (omastar would've cleaned his team if not for a hydro miss against mamoswine but it's a decent replay I guess)
Power:
252+ SpA Choice Specs Omastar Hydro Pump vs. 4 HP / 252 SpD Chansey in Rain: 310-366 (48.2 - 57%) -- 91% chance to 2HKO (after taking a knock off chansey can't switch in)
252+ SpA Choice Specs Omastar Hydro Pump vs. 252 HP / 168 SpD Ferrothorn in Rain: 155-183 (44 - 51.9%) -- 72.7% chance to 2HKO after 1 layer of Spikes and Leftovers recovery
252+ SpA Choice Specs Omastar Hydro Pump vs. 240 HP / 16 SpD Assault Vest Azumarill in Rain: 160-189 (39.9 - 47.1%) -- 42.6% chance to 2HKO after Stealth Rock
252+ SpA Choice Specs Omastar Hydro Pump vs. 0 HP / 0 SpD Keldeo in Rain: 222-261 (68.7 - 80.8%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Choice Specs Omastar Hydro Pump vs. 248 HP / 8 SpD Rotom-W in Rain: 191-225 (63 - 74.2%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery