I have two series of pack - one for fun trips, and one for work related excursions.

The fun series consists of a Golite Gust for overnighters and a small very light North Face day pack I have had for years at about 1800 cu in. It doubles as a stuff sack. I also use waist packs on short rambles.

My work related series consists of a Maxpedition Vulture (2800 cu in). It isn't that great a pack, but it is as tough as nails and withstands a lot of abuse. My work overnighter is a venerable Mountainsmith, 8000 cu in. which has hauled all sorts of weird s*#t for me over the last twenty years or so. It is heavy, but comfortable when loaded.

If I wanted to have just one pack for all purposes, I would acquire something in the 3600 - 4000 cu in range, with the ability to shrink for light loads. The first consideration must be that it fits the body and is comfortable. Everything else is secondary.