When you're hiking, you want to layer so you can shed layers, or add them, as you need to. You don't want to be stuck with one heavy layer that's too warm as you're going uphill.
You want to have a synthetic, moisture wicking base-layer. I have one from Costco that does the job, but I'm looking at getting something like this REI Co-op shirt when it's time for a new one. In the summer I'm usually fine in a loose synthetic running tank top I have.
My next layer is a fleece mid-layer. I use my Patagonia Better Sweater, but there are lots of different types of fleece options out there. I looked at a few brands, Columbia, Arc'teryx, and The North Face, but Patagonia fit me best.
If it's cold enough, I'll even have another insulating layer. For now I've been using The North Face Summit Series jacket. I have a version that's about four years old but here's the current The North Face Summit L3 Down Jacket. I'd really like to get The North Face ThermoBall Insulated Full-Zip Jacket.
And on the outside, I have my water-resistant hard shell. This is usually my Arc'teryx Beta Lt. Hybrid Rain Jacket. It's a pretty great jacket and I love it for backpacking and hiking. And it's on sale as of this post being published! I really wanted my jacket to be made of Gore-Tex® and I loved that this had a detached collar so I could have the collar up but not the hood on, when I needed it. And you want to make sure you size this so you can fit your other layers under it.
For my other "layers" I have my wool running socks. I actually don't think I've ever used them for running, but I use them for every hike I go on. While my favorite running socks are the Balega Hidden Comfort ones, they're too short for me for hiking as mud and rocks will get in them. So my wool Smartwool and Feetures socks work better because they're higher.
My next sock purchase will be these Darn Tough socks. A friend of mine raves about the brand and because I'd like a higher pair of wool socks, I'm planning on getting these next so I can try them out and see if they're worth the hype.
You also want wool or synthetic underwear, and not cotton.
As you can tell, all of my technical layers are upperbody. I have some wool thermal pants I wear under a pair of running pants on cold days, but I do need a pair of water-resistant or waterproof hiking pants for winter and rainy hikes. When it's warm enough, I'm usually just in running pants or capris.
And remember to size your layers so you can fit other things under them if you need to.
Do you have any layering suggestions?