Osprey Fairview 70 Women's Travel Backpack
I've used several different backpacks over the years, and I can honestly say that the Osprey Fairview 70 is the best one I've tried. It's specifically designed for women, so the shape, size, and fit have all been adjusted to fit a female frame. Having a comfortable, well-fitting backpack makes a world of difference, so I highly recommend looking for a female-specific one instead of a unisex bag. I use the 70L Fairview, which has a 55L main bag and a detachable daypack - but there's also a smaller option if you prefer to travel light.
The other really great thing about this backpack is that it opens all the way around, so it's more like a suitcase. A lot of backpacks open at the top, which is a real pain when you're travelling for a long time and constantly having to unpack and repack your stuff.