The lightest, most compact camera which will do the job is the camera that won't be left behind because it's too big and heavy to lug around. Let's consider one camera from each of three types starting at the lightest and most compact.

The domination of the DSLR in the serious amateur market is finally under threat because there are now some other serious options for those of us who just want to take great pictures.
I'm ignoring compacts, superzooms and other snapshooters and suggesting only cameras that will provide the quality of function and results the passionate photographer needs.
The advanced compact
Just about pocketable in a jacket or coat, the advanced compact has a built-in zoom lens covering a modest range, full user control with lots of external buttons, built in image stabiliser to minimize camera shake and an excellent image quality which only falls off in lower light levels.
The Canon G11 and G12 and are superbly engineered and designed examples of the genre. They are capable of producing terrific images in good light with the lower light image quality dropping off somewhat.
On the downside the G11 and G12 have poor ability to create blurry backgrounds (except with close ups and to some degree at the telephoto end of the zoom), slower than DSLR operation and limited low light function pus reliance on the LCD screen or tiny viewfinder for composition.
That said they're great for travel and street photography even landscapes but not for shooting action, sport or indoors under natural light.
The new rangefinder style or micro system cameras
Typified by the Panasonic GF1 and 2 and the Olympus EPL2 this new breed of camera has recently emerged.
Using an image sensor almost as large as the digital SLR and with fully interchangable lenses these new semi compact cameras offer a stylish and just about pocketable (with a small lens) alternative to the SLR.

Image quality can be first class and comparable to basic and mid range SLRs. Full user controls are par for the course.
The excellent lenses including non-zooming primes (we've been zooming to long!!) are compact and pocketable but can be costly.
The GF-1 is a great all rounder but loses it's advantages once you add a long lens.
Ideal for high quality travel, street, botanical and landscape work but at a cost. Not bad for portraits and family stuff but definately not the way ahead for those safari trips or sports.
The digital SLR
Get yourself a basic consumer digital SLR and you've entered a world of unlimited flexibility. Vast ranges of lenses and other accesories enable you to optimise your camera for any shooting scenario.
But beware.

The equipment is relatively large and heavy and before you know it you could be wandering around like a Christmas tree!
Get started with the Canon 1100d with just the basic kit zoom lens and you can add the 50mm f1.8 Prime (non-zooming) for superb low light indoor portraits and maybe the 55-250mm Image stabilised (prevents camera shake) zoom and you have a relatively compact kit which will cover most shooting scenarios.
The best camera?
Before rushing to buy a digital SLR check out this section.
My current best low cost DSLR? It's got to be the new NIKON D3100, a superb machine in every way that really matters. The D3100 combined with the right lens is capable of exceptional results.
My current best low cost DSLR? It's got to be the new NIKON D3100, a superb machine in every way that really matters. The D3100 combined with the right lens is capable of exceptional results.