Now (the grain-free foods) and Go (the grain-inclusive foods) are both really good quality, but Acana is better. If you're going grain-free you generally want a nice high protein content. Now's adult GF formula is 26%, Acana Grasslands GF formula is 32% protein. Now also has three different protein sources (duck, salmon and turkey) but Acana has four varieties that you can rotate through, if switching protein sources is something you want to do.
Here, Acana is also cheaper than Now for the same amount of food, but that's a very regional thing, so I'm not sure about where you are.
Go has their one GF formula now which monkeys mentioned (and of course they would - as it is I always confuse Now and Go, and now Go has a GF, despite Now/Go being the same company... wtf). We don't carry it yet, but it looks pretty good. The fat and protein are lower than most GF foods, but that can be good as some dogs can't tolerate the high protein/fat levels in most GF diets.
But honestly, I'd go with Acana first and see how they fare, as it's IMO one of the best dry diets available.
(Can you tell I'm bored at work and have an itch to talk pet nutrition? LOL)