What about Symbiosis? This ability may seem weird and unrelated to Plasmanta seeing as how its one of the Florges lines signature abilities, but it actually makes sense both concept wise in relation to Gyarados and for real life Mantas. Take a look at the definition of it:
interaction between two different organisms living in close physical association, typically to the advantage of both.
This perfectly fits our Mega Gyarados/Plasmanta relationship, and real life Mantas are actually shown to visit aquatic cleaning stations to get rid of parasites, and in turn giving the fish there a meal. Also, this ability is useless in singles and pretty bad in doubles as well, so its a nice flavor ability.
Other than that, right now I'm liking Telepathy, Anticpation, and Poison Touch