Naked Casein is the minimalist's answer: a single ingredient — 100% micellar casein — with no sweeteners, flavors, gums or fillers of any kind, and it's NSF Certified and independently tested, which is a real mark of quality. You get 26 g of slow-release protein and 5.9 g of BCAAs per serving, with 76 servings in the 5 lb bag. The trade-offs are exactly what you'd expect from a no-additives product: it's unflavored, so you bring the taste, and plain casein is bland and notably thick to mix.
The bulk bag also carries a high sticker price even though the cost per serving is reasonable, and it has about 2 g of residual lactose per serving. For purists who want nothing in their casein but casein — and want it third-party certified — this is the cleanest option here.