Diamond pickaxe with fortune 4 unbreaking 3. If possible, make it so that you cannot add any enchants to it so that it can never gain efficiency. If this is not possible, it probably become OP if you can add efficiency to it.
I agree it would need to be diamond due to the fact of commands++ only working on diamond tools... As for fortune 4... I think it would be way to OP
Assuming you can make a tool that can't have efficiency added to it, A zero efficiency diamondpickaxe would break ores a lot slower than efficiency 5 diamond pickaxe when you are cashing in your ores. You trade time for more mined goods. It could be a fair trade, it just needs some testing.