Max thickness is always the best unless you wanna boost your rubber.
Well, I certainly prefer playing one or two notches under max, and in my experience there's no single best solution for all playing styles and all personal preferences.
Yes, for Werner Schlager, Ma Long, Zhang Jike, Wang Liqin, Kong Linghui, Timo Boll, Vladimir Samsonov max thickness is probably optimal (but I'm not even sure of all of them). Maybe it's not for Matthias Falck, Mima Ito, Gionis Panagiotis, Ruwen Filus, Oh Sang Eun, Tomokazu Harimoto (at this point), and certainly not for Evgueni Chtchetinine.
And not for me. I'm a mix of both-wing looping and direct over-the-table play, and at some point in rallies (4th, 5th ball...) a more direct hit, through the sponge and engaging the full power of the blade, is likely. All that sponge just gets in the way of that power. Spinning's nice, but it's not everything. I play with Rozena (max) now, but I miss a bit of directness on my BH with it and will replace it with FAG1 at 1.8mm in a couple of weeks, for this reason.