I used to have a D70 - now have a D200 - and resisted buying the 18-200 VR until I also had enough money to buy the 12-24. I love the versatility of the 18-200, and it's the default lens on my camera when travelling - but yeah, it's decidedly weak on the wide end (I did some tests comparing it to the 18-70, and that one was superior all the way; sharper, and much less prone to suffering from chromatic abberration), and since I shoot a lot of landscapes, I just knew I needed something to make up for that weakness of the 18-200.
I really don't like changing lenses, or having to carry a second heavy lens - but I do think that 12-24 + 18-200 combo is the best possible lens-setup you can currently get, and I know I couldn't make do with just a single one. Due to the overlap between the two lenses, you'd switch lenses a lot less often than you might otherwise (I used to have 18-70 + 70-300, and that was completely painful), and can usually remain shooting with just one until you reach a good resting point to switch the lenses undisturbed.
Still, if you're adamant that you want to only ever use a single lens, I'd recommend sticking with the 18-70 if you often shoot wide, and otherwise getting the 18-200.
Usually I combine a normal small digital camera (without lenses) and my big camera with 28-200 mm or 70-300 when I go to Africa for example. Focusing on Europe, I probably stick with my 18-70 mm lens.
dont like to change lenses,takes to much time,I take my photos fast my camera has to be back in my purse before the people recognize what I did or the animals are gone before the camera is ready.Thats why I like a good zoom with image stabilization.
till now I carried 2 cameras with me while travelling.
Olympus E-100 RS 10 zoom,38-380 mm shooting 15 frames per second
love this camera very much,perfect for photos of animales, sports.
Olympus 5050 has a very good F 1.8 3x all-glass zoom
this smaller then the E-100 I carry around as a back up,use it for landscape or "party-time"(tele is not working anymore)
Now I got camera Nr.3,a Nikon Cool-pic 8800
10x optical lense 35-350 mm 8,9-89mm 1:2,8-5,2
I'm still testing this new toy and comparing with the Olympus E-100 wich I think is faster then the Nikon.
[ Edit: edited by marlis ]
I have a Sigma 18-200 for my Canon Rebel and I've been delighted with it. I know there's a lot of controversy about it, and it is a bit soft when used full out in lower light conditions, but all and all it's a dream to not have to lug around a several lenses (and miss shots because I need to change). And it's fairly inexpensive. (Note: I have much better luck with manual than auto focus.)
I started out with a D70 and 24-120mm combo. Depending on what I'm doing, that's all I really need. Sometimes the camera of choice is my Canon A620 pocket digi-cam. The deciding factor for what I take with me being subject matter and how & how much I'm going to be travelling.
I can suppliment that combination with:
15-30mm EX Sigma
70-300mm ED Nikon
80-200mm/f2.8 D Nikon
90mm Tamron macro
Manfrotto aluminum tripod
Manfrotto digi monopod (actually would like to have a sturdier one, but it's ok)
2x Kenko teleconverter
set of 3 Kenko extension tubes
SB-600 Nikon Speedlight flash
Vivitar 285HV flash + peanut slave
Alien Bee SB800 strobe flash head + 1 reflective, 1 shoot through, 1 softbox umbrella
2 Elinchrom stands (love these, wish I had one more actually)
I've got other gear and gizmos. But the above is what I use most often. I find that there's not much that I want to do creatively that can't be done with this kit. Going beyond this would only be justifiable if I went pro.
Lately, I've been taking the SB-600, 300mm and the kenko stuff with me. Basically that gives me capability from macro to 600mm in terms of lenses. The flash gives me more range during low light or when I want to use it for creative reasons. But I try to work with out it. The other stuff I use mostly closer to home. As you can see I go for robustness, simplicity, range & flexibility rather than the highest quality.
Wide angle. When in Europe I use my 10-22 almost exclusively.
I have hundreds and hunderds of examples.
I photograph mainly with a D700 (used to be D3 and D300) - and I've found out how often I can get by with simply a 50mm/1,8 lens in many travel related situations. Portraits, architecture, landscape - everything except wildlife and wide-angle "Wow"-shots.
To get the equivalent of a 50mm on a Canon Xti model or any other camera of the smaller APS-C format, get hold of a 35mm lens.
Fixed lenses provide sharper results, with a more "professional" feeling to the out-of-focus blur since their often larger aperture. And the best part about the fixed ones are that they get your mind working (you have to plan more than with a zoom lens) - so you get to zoom with your feet, and think before shooting. Hence, I'd say you learn much more when using a fixed lens.