While your mileage will definitely vary, here’s what I brought:
EMS Fencemender Pants
If I can only bring one pair of pants, these are them. They’re not breathable nor made to be super quick drying but they’re incredibly durable and quite comfortable. I bought a pair on sale and after breaking them in, I went back and bought another for future use.
Under Armour Compression Shirt
I’ve found that using a compression shirt as my true base layer greatly increases my comfort level when wearing a backpack for an extended period of time. It keeps any friction from the straps in check and is wicking so I don’t overheat.
ExOfficio Boxer Briefs
The trip is 3-4 days but you only need one pair of underwear if you have the right pair. These things are comfortable, dry quickly, and don’t end up smelling like you’ve been wearing them for days. Though you’ll most likely have the opportunity to wash them in a creek if you truly wanted to.
REI Lightweight Long Underwear
When I saw this item on the list, I thought about not bringing them because I almost never need thermal underwear for anything. But for events like this, I tend to put trust in the organizer. That said, I only used mine once (for sleeping) but I’m a cold blooded New Englander and tend to run pretty hot. Oh but I did see some of my fellow riders put them on in the middle of rides. Then again, they were wearing lightweight hiking pants too.
Outdoor Research Sequence Long Sleeve 1/4 Zip
This is one amazing piece of clothing. It can handle both hot and cold temps really well and makes an excellent base layer. I wore this over the compression shirt and under my hard shell and was never uncomfortable.
SmartWool PhD Ski Socks
With all the mud, snow, and water crossings (at least on my trip), I recommend bringing two pairs so you can alternate every other day. I swear by the PhD line of SmartWool and these ski sock length versions were the perfect height for riding boots. My feet never felt better.
*EMS Men's Power Stretch 1/4 Zip
I saw many puffy jackets on this trip but I preferred my fleece pullover as a solid insulation layer top. Just something that I’ve had forever and it works really well. I also don’t have to worry about it ripping like a puffy jacket. One guy’s puffy was more duct tape at the end than anything else. The only downside of a fleece is that it’s fairly bulky.
*They don't make mine anymore so I linked to the latest version which is nearly identical but includes a built in hood.