Most Important AR15 Parts

Toe bone connected to the foot bone
Foot bone connected to the heel bone
Heel bone connected to the ankle bone
Ankle bone connected to the shin bone
Shin bone connected to the knee bone
Knee bone connected to the thigh bone
Thigh bone connected to the hip bone
Hip bone connected to the back bone
Back bone connected to the shoulder bone
Shoulder bone connected to the neck bone
Neck bone connected to the head bone

Short answer ... All of them.

Sure, you can isolate one or two key components and say they are more susceptible to cause of failure, but the key to reliability is to ensure that all components, from metallurgy, fault tolerance, to quality control are top notch. I think that distinguishes the difference between top shelf products like a Colt (and others) and those thrown together with lessor quality parts. After all, the original design specs take that into consideration. Use a slightly inferior part and that can potentially have an impact on another component that can lead to failure.

No guaranty that a top tier component won't fail but quality parts have a better reliability factor than those used from seconds bin. It all comes down to what failure rate you are willing to accept and its cost to you.

My $0.02.

Learned an important lesson this weekend about this vey subject. I have a Sig m400 and wanted a free floating handguard on it. In order to install it, I needed to get the pinned gas block off and install a low profile one. Dang it was a chore to remove.
Got it off and installed a spanking new $12.50 steel gas block. I do believe it was made in China for what its worth. I used the same gas tube I took off with the new gas block. I went to the range and wow..I had a single shot AR! It wouldn't eject a fired round. In the rare case it did eject,the bolt wouldn't lock back on an empty mag, but would if I pulled back the handle manually. I looked at the gas tube an it did have soot built up at the gas block end. It was also bent a bit and touching the barrel nut. ( yes, it was installed correctly, but it was difficult to insert " right side up into the gas block. Upside down, it inserted effortlessly) I replaced it with a new tube and now, I have a semi auto again, but it still wont lock open after the last round. It will if I pull the handle back. It wont cycle with Tula ammo either. It functioned flawlessly before I changed out the gas block. I think it'[s undergassed due to an improperly drilled, cheap gas block. So...I just ordered a new, steel, adjustable, made in USA gas block. I think this will solve the problem. Goes to show how one little inexpensive part can stop a good AR from running correctly.