What Is the Best Y Level to Find Ancient Debris?

The best Y-coordinate to find Ancient Debris is Y-13, seeing as you are below the lava oceans and the ancient debris seem to spawn more frequently and in larger quantities. Hope this helps!

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.