Base top:
The base layer lays the foundation for your comfort, seeing as how it is the layer with the most skin contact. The main rule here is no cotton anything. Cotton soaks through quickly and dries slowly. Your base layer's primary goal is to provide you with warmth and breathability as well as "wicking" (moving moisture) away from your body. So depending on the situation, what you want in a base layer will change. If you're going out for an overnighter on a 100 degree North Carolina summer day, you may want a light synthetic short sleeve that breathes really well, such as the Patagonia Capilene 1 Silkweight, but if you plan on spending a winter night above 10,000 feet elevation in Colorado's Rockies, you may want something a little more substantial such as Rab's Baseline Hoody. Both of these are designed to be worn close to skin and will provide you with a similar wicking efficiency.
Base bottom:
This layer could be tights, underwear, running shorts in place of underwear, or any combination of the three. I personally like to bring a pair of tights on every trip. If it's chilly during the day, I can use my tights to provide the extra warmth I need. If it warms up, I can simply take them off. During three-season hiking, I typically just use them in conjunction with a pair of minimal running shorts such as Brooks Sherpa Short III or the GoLite Men's Mesa Trail Running Shorts. Tights are also nice to have at night if you'd like to boost your temp rating.
Insulation:
This is the workhorse when it comes to warmth. Typically non-breathable and multi-purpose in that you can use it to increase your temperature rating in your sleep system. This is always nice to have, even in shoulder season hiking if only to wear at camp or on chilly mornings. And if temps dip below your comfort level, this will warm you up quickly, especially when hiking/burning calories. Most often insulation will mean a lightweight puffy, fleece is another option but is not as light. We like to use synthetic materials like Primaloft or Climashield Apex since they retain warmth even while wet, unlike down. Stevie uses an Arc'teryx Nuclei Hooded Insulated Jacket, while Colton uses the Montbell Thermawrap which is hoodless. Hoods are a good option as they increase potential warmth for very little weight gain; however, they can be redundant if you typically bring headgear such as a fleece beanie. Unless it's extremely cold out, this is usually much too warm to wear while hiking, running, etc., but packs down small so you can store it or swap it out for a lightweight wind jacket.
Wind:
This is maybe the most versatile and easy-to-layer piece in your arsenal (and probably the lightest, too). A wind shirt's main purpose is to block wind (duh), which allows the layers underneath to retain their warming capabilities. Wind steals your warmth, simple as that. You can wear this over your base layer or insulating layer to provide extra warmth. Most running/outdoor clothing companies make a lightweight wind shirt or jacket and they're mostly the same, give or take a few ounces. Stevie currently uses the Montbell Tachyon which is, to my knowledge, the lightest available wind jacket. Colton uses the heavier but hooded Patagonia Houdini.
Rain:
This is obvious, it keeps you dry. What you want to look for here is a nice compromise between weight and breathability. If a rain jacket has no breathability at all, you're going to wet out from sweat alone, and that can be even more miserable than having no rain jacket at all. Unfortunately, some of the more breathable shells out there are heavier. More breathable fabrics include eVent and Goretex varieties, but as mentioned before, these come with a weight penalty. Lighter shells have been known to wet-out in extended downpours so choosing a shell really depends on the weather and conditions you'll face. In the summer, when showers may be short or a nice break from the hot humid weather, a lighter shell performs perfectly. However, in colder weather, you want a shell you can rely on. Hypothermia sucks. For three-season trips, Stevie uses a ZPacks WPB Cuben Fiber Jacket with pit-zips, for packrafting or colder temperatures Stevie uses a Kokatat Gore-Tex Anorak. Pit-zips are a great way to regulate body temperature. Colton prefers to go for a poncho such as the Frogg Toggs Driducks Action Poncho, which may not be as fashionable or formfitting, but is comparable in weight and much more cost effective for the frugal consumer.
And for your viewing pleasure, here's Stevie's shoulder season layering system:
Exofficio Boxer Briefs
Injinji Toe Socks
Railriders Adventure Top
Brooks Sherpa Short III
Patagonia Capilene 4 Expedition Weight Hoody
Patagonia Capilene 4 Expedition Weight Bottoms
Montbell Tachyon Anorak
Arc'teryx Nuclei Hoody
ZPacks WPB Cuben Fiber Rain Jacket w/ Pit-zips
