Depends on what you want to use it for. I keep a pack in my car in case I ever have to walk out of somewhere my car and ego got me they shouldn't have, and because I can just grab it and go if I want to do a day hike--I keep my boots in the car, too. the pack is an REI Halfdome, which is excellent--very comfortable, decent capacity, bladder pocket, and affordable. The civilian packs are very impressive these days.
Lighter, sweater, polypro tshirt, an MRE and powerbars, water, hat, candles, bug juice, sunscreen, socks, garbage bags (el cheapo raingear 'cause I don't want expensive Gortex I never use to just sit stuffed in a pack), nylon tarp, 550 cord, multitool, TP, reading material (Cicero, "The Nature of the Gods", good stoic reading), notebook and pencil, and a ludicrous bayonet I got for my Mossberg 590 and carry around because it's better there than in a closet. I also keep a car kit with a bunch of other stuff.
The basic idea is to be able to walk 40 miles or so and spend a night out in unpleasant but not extreme weather.