Here is a list of every item that can be flung using Fling. The most valuable item, by price, is a tie between Comet Shard and Relic Gold, which can each be sold for 30,000. Only the Comet Shard is legally obtainable in XY, though.
In terms of usefulness / difficulty to obtain, that’s a bit more subjective, but your best (or worst) options are probably something like Sacred Ash, legendary-specific items (Soul Dew, Griseous Orb, etc.), Discount Coupon (only obtainable through Global Link), or Mega Stones, especially Blazikenite, which is not regularly obtainable in XY.