I just went through this same process and ended up pulling the trigger on Seek Outside. I did a ton of reading, watching videos and research on each of them, called in and emailed each company and drilled them with questions. After all of that I narrowed it down to Initial Ascent, Kifaru, EXO and Seek Outside. For me, I was looking for a pack that was lightweight and could handle a heavy load really well.
Honestly, I think all of them--as well as some of the other packs mentioned on here (Stone Glacier, KUIU, Mystery Ranch)--would be packs most people would be happy with and you probably can't go wrong with any of them.
I've used EXO, Eberlestock, Tenzing, Badlands, Kelty, Osprey and now Seek Outside under heavy loads and the Seek Outside was hands down my favorite. EXO was also very nice. There are a few good forums on rokslide about different packs as well that were pretty helpful. What pushed me over the top to pull the trigger on Seek Outside was the following:
1. The fabric they make their packs out of (X-Pac) is a premium durable and waterproof material. This means no need for a pack cover and no blood stains with easy washing. You can actually take the bag off the frame and put it in a front load machine if you want.
2. Their suspension system/frame. Instead of using the same frame idea that is already out on the market and trying to improve it, they started over from the ground up doing their own system. The frames are very well built...they can support 300lbs in vertical load yet weigh only 6oz. They are also very versatile in that you can have multiple pack bags and switch between smaller and larger packs if needed. I ended up doing a custom build and took certain things I liked from a couple different pack bags. But it's nice to know I could just buy a different pack bag and switch it out if needed.
3. Lightweight. Starting at sub 3lbs, all their packs are sub 5lbs.
4. Adjustable frame height. 24, 26 or 28" frame height. Having the ability to adjust your frame to be taller if you get an animal down makes the pack out easier.
5. Very configurable. Not only are they good for hunting, but you could use them for backpacking, pack-rafting, ski mountaineering, etc.
6. Excellent Customer Service. They have treated me very well and are super nice and helpful.
7. Made in the USA.
Speaking from experience, their packs can be a little bit of a challenge to fit sometimes because they have a ton of adjustments built in. But once you watch the videos on how to fit it to you and when you make the right adjustments and it's fitted right it'll probably be the most comfortable pack you've had on your back. I thought the EXO I wore was comfy till I used this one.
Just my two cents. Again, all of the other packs mentioned on here are super nice and you probably can't go wrong with any of them. Seek Outside just checked off the most boxes for me and what I was looking for. I've been very pleased with my decision so far.