Spike Shield. Isn't Water Shuriken physical? Greninja has a higher SpA,
Meanwhile Spike Shield can deal huge 1/8 of the enemies health with good prediction so i'd have to say Spike Shield. --- #RememberTheManticore - SMITE "I'll bait you and outsmart you" - nobody important
Spike Shield by far. Water Shuriken, if it IS a priority move (I haven't seen conclusive proof of that), is pretty nice, but Spike Shield is much, much better. And lol, the worst of the starters gets the worst of the moves. (Mystic Fire is pretty bad, especially since Delphox isn't terribly strong.)
Probably Spike Shield. Water Shuriken is unreliable for damage and I wouldn't be surprised if Greninja can learn Sucker Punch. Mystic Fire is crap for damage and Delphox doesn't look bulky enough to make a good debuffer.
Spike Shield will at least have niche use as a pseudo-Protect, and the damage is a bonus even if it's negligible.