This step requires you to take some time and do a lot of research. Read threads, read BPL articles, etc.
People here tend to be brand loyal (not a bad thing). But it would be difficult to make a decision based on the feedback in one thread.
GG, ULA, MLD, and others all make good packs. And they all have similiarites, and some unique features. Obviously the first step it to determine what capacity you need, or even if you might need more than one pack.
For me, the next critical step is how easy is it for me to live out of the pack. I think this is huge. How does it carry on you back, how easy is it to get to items you need during the day, and when setting up camp. How easy is it to pack. How durable is it. And of course, how light is it.
The good thing is that many of the cottage industry companies will let you return it, if it is not perfect, assuming you have not requested any customized features.
Even if you cannot return it for some reason, most are easy to sell here on Gear Swap.
I have my favorite brand, based on ease of use, and others prefer a different brand, even if they had once owned one of my favorites. It is highly subjective.