18mm definitely. My garage has 2 sheets of 4 by 4 18mm ply just screwed to the wall. Its very convenient, above the shelves and opposite the bench (Just out of sight to the right). Its a rough and ready solution but utterly versatile. All the most frequently used tools within reach. It help's encourage the boys to put the tools back. You can't really see from this angle, but pretty well every square inch is used. Very quick to place things, just drill a hole add a small shelf piece. 18mm allows for a lot of weight to be hung directly and enough depth to hold things screwed with 18/20mm long screws. The chisels and flat files are in a gap between two pieces of pine - old bed boards that have holes drilled to take screwdrivers and long tools, the chisel are in a row separating the long from the shot handled tools. Less used items like spanners, allan keys are nested behind the marking gauges so it is double depth.This looks like both the cleats and took storage is made from 18mm birch ply, thoughts on making this from either some 18mm or 12mm hardwood ply instead? Trying to save my wallet.
And many tools just held on zinc plated tool clips, screwed to the board, new tools take about 5 minutes to add unless they are large and heavy. The lads and I have a challenge as to how much we can hand on it while still getting access to everything. Must have about 250 pieces on it. Most things are side on rather than flat so it produces about 5 inches over the work top.
This may not be to your taste but I've had an arrangement like this in every house I've been in since I was 14 and its stood the test of time.
Although not that neat, its simple and can be rearranged easily to accommodate new thinking, with all tools grouped its easy to find things.