I've found 1/72 to have a pretty good selection, I would guess it is pretty close to 1/48 as I found 1/48 has some kits I can't get in 1/72 and 1/72 has some kits not available in 1/48. Then again there are a few kits out there in 1/35, 1/32 you can't get in the smaller scales.
What I have found is helicopters in general have pretty poor coverage unless you are doing one of the major US military helos (UH-1, AH-1, UH-60, AH-64, CH-46, CH-47). You might want to look at your particular interest and see who does them and in what scale.
There are some real odd ball fixed wing kits out there, sometimes you even have multiple kits of the same experimental plane in different scales but there are some very common helicopters that are difficult or impossible to find in any scale.
I went with 1/72 because it seemed to me to have a little better selection of civil helicopters, although I am having a tough time finding a Bell 206 / OH-58A in 1/72.
As far as the second part of your question Italeri seems to have filled the helicopter niche better than anyone else, followed by Revell Germany. Heller offers a few that nobody else has.
Trumpeter and Academy have picked up the larger scales. Again except for one of the primary US military helos you probably won't have the luxury of shopping for the best kit, you will just have to take the only kit available in that scale.