In pokemongohub’s database Suicune has Extrasensory/Hydro Pump marked as it’s best moveset where Extrasensory is 10dps in 1.1s. It also states Hidden Power is 9.33dps in 1.5s. However, the database doesn’t seem to take into consideration the difference if you’re lucky enough to find a Suicune with a water type Hidden Power. If I’m not mistaken the water type Hidden Power should be the best fast attack for Suicune. Correct me if I’m wrong?!?
You’re Right. A water type Hidden power is stronger than Extrasensory due to the benefit it receives from STAB. (Same Attack Type Bonus)
Hidden Power Rock is also quite good for the niche Ho-Oh/Moltres/Charizard battles.