There's nothing wrong with the most common medicated feed, one that contains amprolium. It is not an antibiotic. If one has problems with coccidiosis it's a lifesaver.
As for what's the best feed for a flock, that's going to vary by owner. I prefer to use a feed that has more protein than the usual layer flock. I feel my birds do better on 20-22%. So I regularly use Blue Seal's gamebird crumbles, Purina's Flock Raiser, and if for some reason I'm in SE MA I'll get Ventura Grain's gamebird grower. I get any of those brands freshly milled - Blue Seal directly from them in Milford NH, Flock Raiser from Erikson's in Acton and Ventura's from their mill in Taunton. Ventura's is the only feed I've found anywhere remotely close that actually includes animal protein (fish meal). All 3 of these feeds have lower calcium than layer which is better for any chicks, molting birds and males that I may have. Oyster shell on the side for layers.
Organic and nonGMO would be nice, but I feel the higher protein is more important for my flock. The cost is half the organic, which is a definite plus when one has 50 birds. Unfortunately if you really want to stick to organic you'll have to continue with Green Mountain, or see which other companies my deliver to you.