I currently use a Lowepro Trekker AW2 as my main camera bag. The main kit I carry in the bag at all times consits of a Canon 5DMK2 body, Canon 17-40mm f/4L lens, a Canon 70-200mm f/4L Canon Lens, and a Canon 50mm macro. I also keep numerous filters and a Canon 550ex flash gun in there along with a remote cable release, hotshoe spirit level and a few other small items.
Around 12 months ago I had a rethink on what camera equipment I actually needed with me in the camera bag. I used to also carry a Canon 100-400mm f/4.5-5.6L, a 50mm prime lens and a spare camera body. However after carrying a similar set up for the last eight years or so and starting to get a bit of back pain I decided to shed a bit of weight from the camera bag and out came the Canon 100-400mm f/4, the 50mm lens and a spare body. I only put the spare body in there if I am doing a really important commercial shoot that couldn't be re-shot if the worst case happened and my camera body failed.
The lens I use 99% of the time is the Canon 17-40mm f/4 lens. I love the versatility of this lens and it practically lives on the camera. Each piece of equipment or accessory has its own place in my camera bag and I always put that equipment back in that particular place. This is purely due to the fact that with landscape photography you do often have to work very quickly in order not to miss the light so knowing where each accessory is in the bag is extremely important to me. Even my lens is kept with a filter holder permanently attached to save time setting up.
I use a selection of ND grad filters from both Lee & Hitech along with the Lee Big Stopper 10 stop ND filter for really long exposures. I have both Hard & Soft ND Grads from Lee and use the hard grads 90% of the time. For the Hitech filters I have a set of three of their ND reverse Grads along with a set of three of their full ND filters. I also carry a Heliopan 105mm polariser with me in my bag. I have always enjoyed using ND grad filters to control the exposure in camera rather than relying on post production and this suits my personal workflow much better than blending techniques, especially from the time saving aspect.
At all times I also keep a few cleaning accessories in the bag and always have a bit of water and some food (usually chocolate bars).