I think I've grown almost every montipora available in the hobby since about 2000.
Slowest of all/most difficult is the "digitata" referenced by
@Pedoconfuego from Adam
@Battlecorals that he called the Metal-polyp monti. I still have it in several frags and the remnants of this "colony" but it is perpetually unhappy. I have yet to grow anything I would call a robust colony of it after maybe 6 years trying?....this is the closest I got (maybe 2.5" dia) and it has regressed a lot since then. It is not the same species as any other digitata/samarensis in the hobby:
The rest of them all hit a "critical mass" of around 3" in diameter and take off. Orenji is quite slow, but I have about a 6x4" piece that can withstand a few chunky frags from the edge each year.
Orenji a couple years ago. it covers 3 discs and overflows around the edges now:
Beach bum, crazy T, JF halo, JF Outrage, Appleberry, verrucosa and any other really knobby danae/verrucosa type montis with hard skeletons come next, but they all seem to take off over around 3-4" in diameter. Some are very weird and really do best in very low light levels. The ones that withstand higher lighting like the appleberry and verrucosa grow a little faster.
Tyree undata and Jedi Mind Trick grow faster than those, but do take a long time to hit critical mass, and for even 1"+ frags to encrust and look good. Jedi Mind Trick seems a good bit faster than the Tyree for me.
The UWW Rainbow Sunburst, and the CC Pop Rocks are both more traditional encrusting montipora that are quite slow compared to the M. tuberculosa types (i.e. superman, poker star, etc.). They are faster than any listed above though.
CC Pop Rocks under blue LED...sorry my camera/photoshop skills aren't great compared to the Facebookers. It is orange with green, yellow, and darker purplish flecks:
Sunset is finicky for me too, though not terribly slow growing. Some people seem to do really well with it though.
Hope that helps?
Tim