After beating your Starmie, look for the following CP values. These values are those of a Starmie with perfect IV's.
Starmie has 210 attack, 184 defense, 120 stamina. As a Raid Boss, it has a CP of 17384.
Starmie is a Water-type and a Psychic-type. It has access to the following moves:
other viable counters
For other Grass-types, Venusaur, Meganium, and Exeggutor are the best choices.
For other Bug-types, Heracross and Pinsir are best. For the Ghost-type, Banette and Gengar are best. With Gengar and Heracross, be wary of Psychic-type attacks.
For other Dark-types, Umbreon and Absol are best. Absol can also be used with Bug-type and Electric-type attacks.
For raw damage, Salamence, Rayquaza, and Dragonite are best. If you use them, be wary of Power Gem.
Finally, Lanturn, Ampharos, and Jolteon are the best of your other Electric-type options.
IMAGE CREDIT: The Pokémon Company