Well basically most water types.
>252+ SpA Sunkern Solar Beam vs. 0 HP / 0 SpD Hippowdon: 252-296 (70.5 - 82.9%) -- guaranteed 2HKO
252+ SpA Sunkern Solar Beam vs. 0 HP / 0 SpD Keldeo: 210-248 (65 - 76.7%) -- guaranteed 2HKO
>252+ SpA Sunkern Solar Beam vs. 0 HP / 0 SpD Mega Gyarados: 152-182 (45.9 - 54.9%) -- 62.1% chance to 2HKO
252+ SpA Sunkern Solar Beam vs. 0 HP / 0 SpD Azumarill: 230-272 (67.4 - 79.7%) -- guaranteed 2HKO
Etc.
Full list:
Hippowdon
Mega Gyarados
Keldeo
Azumarill
Mamoswine
Manaphy
Politoed
Diggersby
Kabutops
Quagsire
Slowbro
Manectric (of all things)
Absol
Mega-Blastoise
Crawduant
Rhyperior
Krookodile
Sharpedo
Starmie
Dugtrio
Omastar
Suicune
Vaporeon
Gorebyss
Slowking
Milotic
*over 50% chance of 2HKO
Smogon Damage Calculator