These staples should last you many years and be usable in almost all your outdoor persuits, from summer hiking to fall backpacking, to snowsports, and even a bit of light summer mountaineering.
Without further ado we'll start from the bottom to the top.
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Wool socks
Wool stays warm and wicks moisture when wet. As long as you've got breathable shoes wool socks can be comfortable even in the summer. As an added perk they're nice and cheap. -
Softshell pants Tough, breathable, and windproof are the trademark characteristics of softshell pants. They're typically not insulated and so are suitable for warm weather, but their wind resistance makes them plenty warm when the mercury drops (especially when worn over a long baselayer).
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Tech T-shirt Something most people already have these days. A technical shirt is synthetic and very breathable. It doesn't have to be skin tight, but should fit very close to the body.
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Incompressible top Warm and breathable, but not wind or water resistant. This insulating layer should be incompressible so that it can be worn under other layers without losing its warmth, fleece is a popular choice here. This layer should have a slim fit, snug but comfortable over the tech T. I call it a "top" rather than a jacket because this layer can commonly be found as both a pullover and full zip jacket.
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High-loft jacket This is where some real warmth comes in. This jacket will probably be too warm to wear while active in most climates, but is essential when stopping to rest or around camp. It should have more of a straight fit that can layer over the incompressible top but isn't so loose as to let in cold air when worn directly over the tech T. Duck/goose down, synthetic down, and some newer synthetic "fur" materials are all good choices here.
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Hardshell jacket This will easily be the most expensive item on the list, so you may want to swap it for some cheaper alternatives (especially for outfitting children) that I'll list later. Ideally you'll want a helmet-compatible jacket made from 3 layer waterproof/breathable fabric (like gore-tex). The hardshell is your real protection from the elements it protects from wind, rain, and snow. Although it isn't insulated, you'll often feel much warmer with it on compared to only your previous layers. This jacket goes over everything above, so you'll likely want to size up.
Alternatives:
- If you're going somewhere you know will be dry, and you know your body isn't going to be rubbing up against the ground or trees or rocks: A windshirt (size it to wear under your high-loft jacket)
- If you're going somewhere dry, but you need some durability (skiing, climbing, etc): A softshell jacket
- If you're going somewhere wet, but don't don't need any durability: A 2 layer rain jacket
Ultralight gear is awesome for keeping weight down when backpacking. But what you save in weight, you pay for with increased price and decreased durability. If you're building out your core essential outdoor clothing, I'd avoid ultralight items in favor of more all-around options.