All 3 have their place, IMO. I own multiple pieces from all 3.
Sitka - Durability and quality is unmatched. Guys have hundreds of days on timberline pants and jetstream jackets and they barely look used. Plenty of online reviews back that up. I’d rather be wearing Sitka if I’m cutting wood, setting up tents, getting in/out of side by sides, etc. Sitka has the best re-sale value, it’s the swaro of camo. Cons, price, slightly heavier across the board and for whatever dumb reason, still a very limited merino wool line.
First Lite - Best merino selection and best base layers, no doubt about it. Best solid colors too. Durability of their outer layers has always been their knock, especially with their 1st/2nd Gen stuff. Which was to be expected.
KUIU - Their shit is light and fills the “hardcore” backpack hunter void really well. When I got this Kenai jacket in the mail 5 years ago, I thought no way is it gonna hold up. It’s still goin strong. My guide pants are still goin strong. Knee pads in KUIU’s pants blow and are too thin.
All 3 have their place. Sometimes I’m just huntin cows from the truck and need durable and strong over lightweight.