For me the 150 Nubatama Bamboo. Fast as hell and leaves a good finish too. Not even a fair contest with the other stones mentioned. The first couple times you use it you will be SHOCKED at how fast that burr comes up. Adam this is one you gotta try. The 320 Bamboo has been reviewed favorably against the Beston 500, but the 150 is something I like even more. Just the right mix of fast AND leaving a finish just slightly coarser than the 400 Chocera, so you can easily go from the 150 Nubatama Bamboo to a 1k stone - or even the Green Brick Naniwa.
In general for coarse stones, I like to use diamond plates and use the coarse stones to remove the diamond scratch pattern. Best of both worlds. You get the flatness of a lapping plate and don't put as much wear on the stone so it stays flat during the session. The 150 can be lapped easily with the Atoma 140, making this an excellent low grit 'team'.
When you go to the coarser Nubatamas - 120 and especially the 60 and 24 grit, these are extremely fast metal removers yet hold their shape quite well, but you need something to get those scratches out. For example the Aratae or XXC (24 grit) - use the Atoma 140 for scratch removal
Seriously. You can use the 64 and 24 grit Nubatamas as stone flatteners too since they hold their shape well.
The Naniwa Synthetic Ohmura cuts quickly but dishes quickly too so it especially should be used with a diamond plate as I described above. Especially for single bevels, a soft coarse stone can get you into trouble especially regarding producing wandering shinogi lines and excessive unwanted rounding of flat single bevels. This is also true of the Natural Ohmura, with the natural Ohmura imparting a high contrast coarse finish. People use it as a flattener for naturals, but my personal preference is still a diamond plate for that too. It was used historically before diamond plates were around.