hp fire is only really useful on one thing
How do you get that? Protean makes it useful on anything that is 2x weak against it, not to mention it OHKO's both Scizor and Ferrothorn at full health.
From my experience, basically anything that is at 60% or lower HP (with a fire weakness), which is when your Greninja would come in to clean up, is getting KO'ed by HP Fire.
Not to mention he can punch a good hole in both Klefki and Skarm, who are both pretty common leads. (Skarm not so much anymore, but still)
Also, it's been said before that U-turn and Dark Pulse both provide nearly the same coverage coupled with Hydro Pump/Surf and Ice Beam, so running both would be a waste. It really comes down to whether you want raw power or momentum when deciding between the two.
Basically the moves I've seen on Greninja are Hydro Pump, Ice Beam, and U-turn. I've run into a few Spikes sets, and I've seen a few HP Fire sets. What I haven't seen on Showdown are sets using Dark Pulse, because it's made slightly redundant by U-turn.
IMO, the coverage granted by a moveset of U-turn/Dark Pulse, Hydro Pump/Surf, Ice Beam, and HP Fire is to great to pass up. If you have something else on your team to deal with Ferrothorn and Scizor, then you could opt for Spikes to set hazards and troll electric types. You could also go with HP Electric, which will hit bulky water types, as well as provide additional coverage for flying types without changing you into one of the worst defensive typings in the game.