In order to get the lowest amount of carbs in your protein, you need to pay attention to the following factors:
1. Whey Isolate vs concentrate. The concentrate version has lactose, which is essentially a kind of carbs. The Isolate has no lactose at all, and consequently considerably less carbs.
2. Choose a product with no sugars added, and with as few additives as possible.
Personally, the "cleanest" protein product I was able to find in recent years, is the Isolation by N2BM. It has no additives, and it is pure isolate - so no unnecessary carbs there! Besides, the taste is amazing.