I am currently using an osprey 26+6 backpack. Not the most comfortable carry, but works for me. I can fit a weeks worth of clothes without expanding. Like the clamshell opening.
In the past, I have used an Ogio Metro backpack. Similar capacity, or even more. Lots of pockets and organization. No clamshell opening, so I typically used an Eagle Creek garment folder to easily slid the bundle of clothes in and out.
For me it depends on the trip, preferably a bag as light as possible on its own.
For a 1-6 week trip with a lot of walking, it’s: Mountain Hardware JMT 35L
This is my favorite overall right now. It’s an internal frame pack in a rational shape for carry-on use on planes and trains (not too tall, or strongly curved, or lacking a flat bottom like some trekking packs). Weight is just under 1.2kg (2lb 10oz), so it doesn’t take a lot of the limit for the bag itself. And it’s very comfortable to carry. It has well-designed cinch straps to keep it close to your back. It’s usable, cinched down, as a day pack for hikes at the destination. And it has very stretchy bottle holders and a stowable stretch panel on the back, so it’ll easily carry 4 tall 1L bottles and a big jacket on the outside. I use lightweight packing cubes in it since it loads from the top.
For more space, longer trips, or when I need to fit my camera kit, I take a ~10 year old: Patagonia Headway MLC 45
It has been on more than a year’s worth of travel with me over the last 10 years. It’s easier to pack, and looks nicer than an outdoor pack and is still only ~1.5kg. But with no belt nor frame I don’t want it on my back for more than a few kilometers at a time fully loaded. I’m glad to see the current MLC has cinch straps and a belt.
For trips under 1 week I’m likely to pick: Fjällräven Greenland Top Large/30L
It’s simple, 800g (1lb 12oz), and is a good shape to fit under most airline seats as long as it’s not stuffed. There is no organization, it has few pockets, and the bottle pockets are skinny and don’t stretch. But it’s lightweight for its volume, has a good structure on its own, and it has held up well as both a weekend bag and for carrying picnics into the woods on a hike.