Any gun used against an American in war.
So you're saying you cannot learn from and respect your enemies? While there is no respecting terrorists, there is no point in disrespecting common soldiers even if they serve a nation that is opposed to your own. They're all fighting for the same things you are.
What did I learn from my early-production Arisaka 99 (recovered on Okinawa in 1945 by my grandfather)? I learned the Japanese built a fine rifle with many innovative features. As a bolt action, there is nothing wrong with it, and I would certainly trust my life to it. It was simply outclassed by the M1 Garand, however.
What did I learn from my 1943 Mosin Nagant M91/30? I learned the Russians could also make a fine gun, and that Russian guns are freaking brutal. Given that the average size of a Russian soldier was a bit smaller than an American at the time, the thought of them carrying and repeatedly firing this massive rifle that kicks hard is worthy of my respect. I wouldn't want to use it as-built, and put a recoil pad on it.
What wouldn't I own?
AR-15, unless it's one with a piston upper. Its only feature you cannot replicate on a more reliable, less expensive semi-automatic firearm is its extreme accuracy. Then again, this also varies depending on the quality of the individual upper, anywhere from dime-sized to 2 MOA, and 2 MOA is where Saigas sit. It's just too much money for a problematic platform, particularly when other cheaper, more reliable semi's are good enough to fill its role. They're accurate enough out to the effective range of the round.
Glocks. I'm just not a fan.
Any gun in 9mm. I don't believe in 9mm. It just doesn't have the stopping power of .45 and .40.