I use a couple of packs, depending on where I am going. If I have to take a lot of stuff, I use my old Marine ALICE pack. I can tell you from experience, you can live out of an ALICE forever. For moving fast and light, like on my bicycles, I use one of several daypacks I got from Walmart for around $14.00. For the money, they can't be beat. For extended long hikes, like over-night, and when I don't want the weight of my full ALICE Pack (50+ pounds, easily), I have a hunting pack I also got at Walmart, that holds everything I need for an overnight trip, or even for a few days of roughing it, and it even straps on my Kayak. I seldom use a tent, so that saves a lot of space. I mostly use my Jungle Hammock, or a Bivy Sack to sleep in, or sometimes, I just sleep on the ground. I get my food in the woods, and purify water from the streams, if I use up my 4 canteens worth. I don't take much with me, preferring "In-Country" supply as much as possible. If I needed to, I can make a shelter from local materials.
Aside from my fishing rod, and flies, the most important thing I carry is several knives. As long as I have them, I can deal with anything that comes up. And, in an emergency, I do know how to flint-knap....