There isn't a specific tracking marker that will give the best results with our tracker - we aren't fussy. In most cases you shouldn’t even need tracking markers if you have texture in the surface you want to track.
Anything that has a strong contrast with the background, edges perpendicular to each other and distinctive texture will usually work. White crosses on a dark background work fine for most people. Just keep in mind that Mocha is looking for texture, not individual feature points.
It's worth noting that Mocha tracks using a luminance image (unless you choose "Auto Channel", in which case it selects the color with the most contrast), so you need to make sure your markers have a strong luminance difference with the background.