For long hikes I am very fond of a backpack. It gives me freedom to move around easily and the extra weight is at a comfortable place. I used to have Lower SlingShot 100 AW (All Weather), however, with first getting my D700 and later my D800, I had to look for a larger backpack. The SlingShot had the advantage that in one swing I could take out my camera and have easy access to the accessoires.
Nowadays, I use the Lowepro Transit Backpack 350 AW which is almost as easy to use as the SlingShot and at the same time offers more space to take several lenses with me. The additional advantage is that I can also take my MacBook Air and iPad. I am also able to attach a Bottle Pouch to take a bottle of water.
Last year I also took it to Barcelona. For strolling through the city it is very convenient, however, in the metro, where I almost got robbed, I felt less comfortable. So, now I use a Lowepro Messenger 180 AW which I can carry in front of me, to keep an eye on my equipment. I can also carry it on my side or back. It is very spaceous, so no problem taking my D800 and several lenses, and my iPad. When I take the car to a photoshoot I usually take this bag.
In case I only want to my camera with one lens I take a simple toploader.