I cannot think of a better watch in production under 40mm and I am including Voutilainen's Vingt 8 here. Why I said under 40mm, RW Smith Series 1 is 40mm and Charles Frodsham's is 42mm and they are both terrific offerings but are a lot more money!

The very rare RW Smith Series 1 with Roger Smith's improvised and improved Daniel's co axial escapement

The immaculately made Charles Frodsham spotting Daniel's double impulse escapement