For me, this question all depends on what type of event or place we're going to be walking around and is probably why I've accumulated so many lenses...LOL If there was one-all-working lens, then I'd get rid of everything and only have that.
Here is what I do.
First of all, I always keep the grips on the bottom of my bodies because it provides me with a much more comfortable grip, and also doubles my battery life so I never have to worry about running out of battery juice.1.
If we're going to be outside, and if it's light out, then my ideal lens is the Canon 28-300L f/3.5-5.6 USM IS
- This lens is perfect from wide angle all the way out to 300mm. It focuses fast and has image stabilization for great hand holding ability. Only problem is it is very heavy. Luckily for me, I'm still somewhat young (35) and am in pretty good shape (Dont look at my beer gut)
Remember, this lens is so big, it draws alot of attention, and can sometimes not be what you want if you're trying to be inconspicuous or not draw attention to yourself. If using an on-camera flash for fill, this lens is out as it is so long and big, the flash will always cause a shadow on the bottom of the image. But then again, who the heck is using an on-camera flash with this camera?
With the higher f-stop, this lens wont work indoors or in low light conditions, so it's only for outdoors and in good lighting. Its weight means you almost have to have a grip on the camera to hold it comfortably. 2.
If I'm going to be inside, then my ideal lens is the Sigma 24-70 f/2.8
- This lens is big, 77mm front element, and allows in a lot of light for low light subjects or situations. It focuses fast and has a great minimum focusing distance as its also a Macro. Ideal for indoor, low-light photography and can get you by without drawing too much attention to yourself. The 24mm gets you plenty wide, but I sometimes find the 70mm to lack in length to get me close to the subject. This is probably the lens that stays on my 5D the most. This is also the lens I would like to upgrade to the Canon 24-70 f/2.8 because of the Canon's legendary speed and shaprness. The Canon also has a better hood, providing better protection from light interferance and glass protection, but also costs $500 more than the Sigma, which is why I own the Sigma
With this lens and it's hood, your on-camera flash is again useless.3.
There are certain times where holding a big camera with a big lens can get you asked to leave or will cause too much attention. For instance, we tried to walk into The Biltmore in North Carolina with one of the cameras with the grip on the bottom. The security stopped us saying "No professional cameras!" I walked outside, put the grip in my back pack and walked back in and they didnt say anything
? Same camera, just looks smaller and more consumer like. With the grips off, we can sometimes get away with sneaking the cameras into places that say "NO PROFESSIONAL CAMERAS" like zoo's, some museums or events that we might not have press passes to.
So when we're trying to be inconspicous, we use the cameras without the grips on the bodies and throw on a basic Canon 28-135 f/3.5-5.6
. This lens allows for great versatality and allows you to use the on-camera flash, something that I hate, but again, putting a big flash on will draw attention. This lens is also pretty slow to focus and you'll need good light to get good images. If they made a lens like this that was in a fixed f/2.8 to allow for low light capabilities, I'd own it! Add Image Stabilization to it, and I'd have two and they'd probably never come off the cameras.
That's my walk around lenses. I'd be eager to see what others use to get by. I know Luca swears by the Canon 24-105L f/4 USM IS, but again, I try to only buy fixed f/2.8 lenses as they're usually so fast to focus.