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XCThere are so many scenarios and answers to this question! I'll answer a few things I feel strongly about.
Merino wool. It's luckily no longer a lesser known material. You can find great base layers from Smartwool, Icrebreaker, and I'm sure there are tons more. My briefs, thermal underwear, t-shirt, socks, and liner gloves are all merino wool. It's comfy, is a material that still maintains warmth when wet (this is huge when sweating on the uphill), and DOESN'T STINK (relatively). Try some regular underwear versus merino wool underwear experiments...the stank just isn't there. This really is important when you have to spend weeks, or even just a few days, in a pair of underwear.
Cap. Sunglasses. Whatever your style. Don't EVER forget sunglasses. You have to protect your eyes and you never know when the sun will come out. You don't want to hike in goggles.
I mainly wear a my t-shirt, varying thickness of longsleeve shirt, and a hard shell on the up. This is where your style comes into question, there are lots of great options. Just don't sacrifice looks over weight and quality.
Pants. I like baggy. My resort pants are pretty loose. It's hard to find a good compromise when touring. You just can't do baggy. It's an outright liability when climbing (mainly just when using crampons). You can do baggy but bring a strap to cinch your cuffs down when climbing with crampons. This is my "compromise": https://www.moosejaw.com/product/mammut-men-s-haldigrat-hs-pant_10446585 (careful, the bib will catch your stink FAST, I don't even use mine anymore).
I have a solid hardshell and softshell pant. But if you have to choose one, I'd go with a 3L weatherproof hardshell.
I always keep a small/thin down jacket in my pack. It comes in handy all the time.
I could go on forever. Hope this helps!