What Is the Best Y Level to Find Ancient Debris?

To be more specific, Ancient Debris is found most frequently when the player's y-level (as read on F3, for example) is 13 or 14.

In an idealized 1x2 tunnel, the player exposes 8 blocks for every block they move forward That's a block each for the ceiling and floor of the tunnel, and three blocks for each of the y-levels the player is moving through (with one block being mined and two on the sides). So, each of the y-levels that the player is moving through are 3/8ths of the exposed blocks, and the floor and ceiling y-levels are 1/8th each. As @TimmyJim said, surveying y-levels for Ancient Debris shows that Y=15 yields the most, with nearby levels decreasing at similar levels. There does seem to be a small but significant higher frequency on the levels greater than Y=15 than less than, so Y=14 will yield a very small amount more than Y=13, but that difference is so slight it is not important. Minecraft Wiki user Awikhax provides this chart:

So, as long as either the lower or upper level that is being mined is at Y=15 (that is, NOT the ceiling or the floor of the tunnel), you should be at maximum efficiency, ignoring lava pools and the like. This means that the player y-level can either be 13 or 14.