+spd -def vs +atk -hp young summer tiki

+Atk for the enemy phase builds, and +Spd for the player phase builds. If you want to play it safe, I'd say the +Atk would be best, since S!Y Tiki's spd stat is already good enough to prevent doubles, and unless you're willing to invest on her spd stat so it goes over 43-45 to consistently double, keeping it at 37 is good enough. There's also the fact that -def hurts her more than -hp. If you're really unsure, then you can always keep the other one sitting around as a backup if you ever do change your mind, or if something big happens.

+ATK for sure.
Congratulations on acquiring Y. S. Tiki.
She is really difficult to get.

I'm surprised by these answers. +spd -def is my preference by a wide margin, especially on a flier team. I made a topic examining her IV possibilities; if you want you can look it up on my profile

I don't like -HP better than -def, especially without merges, because sustain builds become harder to play and she can get into trouble against blue dragons.