I have a lowepro fastpack 250. Decent size, holds my d80 + 18-200, 10-20 & 70-300, any of them can be on the body. Little space up top for snack and water, the laptop area is good for just that or a book / maps. The biggest advantage I like is that you can open just the side and pull out the camera without opening the whole camera area. It's not the most comfortable bag, but I've done plenty of hiking with it and it's not awful either. Fits for carry-on, under or overhead.
I like it, but I'd also be open to try something else if I happen across something else. Also looked at the Kata's & tamrac. Settled on the fastpack because it had more room up top for non camera related stuff when carrying-on flights, or room for a light jacket or shedding layers while hiking (though again, not the most comfortable for hiking 10 or 15 miles, but not the worst either). If I didn't want the extra space up top, I liked the kata sling 3 in 1 or some of the tamracs felt ok.