Think Tank Streetwalker Hard Drive Backpack
I must say, that what I like in a bag may not be for everyone. My main bag must have the following qualities:
1. It must hold 2 SLR Bodies, One Long lens, 2-4 short lenses, 2 Speedlites, ST-E3 Transmitter, at the same time.
2. It must be able to hold a 15″ Laptop Computer.
3. It must be small enough to fit into an overhead bin of an airplane. It needs to be mistakable as a normal backpack, not a HUGE photography bag.
4. Accessory pouches are a must. Thumb drives, batteries, memory cards, external HDs, cables all need to be available while on the road.
5. Dual Zippers are a must. Nothing irks me more about a bag that only has one zipper per opening. The zippers must be extremely tough because they will typically be the first thing to break.
6. It must feel good to wear. This comes with the right width of the should straps as well as the padding.
Think Tanks Street Walker HD fits this better than another bag that Ive seen. They make tremendously awesome gear and I cannot recommend it higher. 
Purchase Think Tank's Street Walker HD Backpack
Think Tank Lens Changer 75
Its essentially a big lens pouch. I use it for both the Canon 70-200 2.8 IS L and the Canon 100-400 f4.5-5.6 L IS , both very big & heavy lenses. The reason you might want to think about one of these guys (or the smaller ones) is your bag may be already full and you want to bring just one more lens. No problem, throw it in this pouch and attach it to the underside of your backpack. Great little investment. 
Purchase Think Tank Lens Changer 75
