My go-to is the Peak Design 30L. Admittedly, it's a bit large, really, for EDC use, but as a Do Everything sort of backpack it's been perfect for me. I've used it for international and domestic travel as a carry-on, as a hiking backpack for day hikes, a weekend/2-3 day bag for short road trips, work backpack when I need to go to meetings in the field, etc. That said, I also will sometimes carry a good bit of camera gear so that is why I originally landed on that bag, though I DO use it without carrying any camera gear as well and have loved it.

I'd probably be OK with the 20L version too, but the 30L has the expandability I wanted to be more of a Do Everything bag.