for me it depends on the trip, short trips on fast moving water I have a dimension typhoon that works great, but it won't hold much gear. For long trips on flat water I'll use a Perception Carolina, it's a touring kayak and can pound out mileage with ease. I also have a Perception Sundance that is in between, it's sleek enough to work well on flat water, but short enough to turn well when I need to.
AngrySparrow, as usual, took a different approach and got one kayak that is diverse enough for any trip. His Native Watercraft MantaRay has been through class III rapids and made a 70 mile trip on flat water. It holds a ton of gear, is comfortable and very well designed. I've tried it out and I want one (I may have a severe gear habit ... maybe). For long trips I'm taking the Carolina, but the scupper holes and seat on the MantaRay make me jealous.
Longest trip I've done is 3 days, but we are planning a 10+ day trip and if I can't fit what I need in my Carolina I'm sure I can stuff some things in the MantaRay
To keep everything dry we use standard dry bags. I use several different sizes and nest them. I find that water will always get in, you take something out, use it, it gets wet, then you put it back in the bag. But if you nest the bags you can keep the stuff that has to be dry separated from other gear that may have gotten wet. I keep my hammock, tarp and sleeping bag in one 10L dry bag and then nest that inside a 30L along with several 5L bags. If the tarp gets wet I'll move it out to the 30L and my hammock and sleeping bag still stays dry.