And this depends on what focal lengths you need to have available, and how fast the lens? There is no perfect lens for all needs. The Pentax DFA 24-70mm f/2.8 will give you some wide angle at its shortest setting with APS-C use. It is made for FF use, where it would provide a much wider angle. Of course no problem using it on APS-C bodies where you'd get less wide angle. But with its hood on, it weighs around 2 lbs and is 4.3 inches long without the hood, has a front circumference of 82mm for any filter you might use, and has around $1,000+ price tag. I'm sure a good multi-coated circular polarizing filter of 82mm will be quite costly also. Distortion is very high at 24mm as is wide open vignetting (darkening of image corners), which is to be expected of such a lens.
But do you really need to have f/2.8 speed? The beauty of APS-C is moderate size. The DA18-135mm is only 3 inches long without hood, just a 62mm filter size, weighs under 1lb. with hood, and is priced well under $500. Wide open between 18-70mm, already a much greater zoom range than the DFA lens, it will be at f/3.5-4.5, or about one stop slower than the DFA lens. This means it will need to have the ISO doubled in lower lighting to get the same shutter speed, which will increase noise. For example, If the DFA lens wide open can use ISO 1600 to get the needed shutter speed, the DA 18-135mm lens, wide open from 18-70mm will need ISO 3200 to get the same shutter speed. Your kit lens, however, and many other shorter zoom lenses, will already be at f/5.6 wide open at 55mm, which will require ISO 6400, where noise increase with the K-5 will definitely be significant.
The DA 18-135mm, however, exhibits somewhat lower vignetting and distortion at its widest angle, which is a lot wider than the DFA lens, and at 24mm, which is the DFA's widest, this DA lens comes in at very low distortion and vignetting. What many of us do is get the zoom lens that best suits our needs. It might not be the fastest available, unless that is what we need. If not the fastest, for low light and other fast-aperture needs, we obtain one or more fast primes, which are faster than most fast zoom lenses. Like the FA 35mm f/2, a FF based lens, or the cheaper DA 35mm f/2.4 prime lens. For moderate telephoto there is, of course, the superb FA 77mm f/1.8 Limited.
If requiring a shorter pro-style zoom lens of f/2.8 aperture for APS-C, there are alternative lenses like the very nice Sigma 17-50mm f/2.8 EX DC. It is 3.6 inches long without hood, and weighs just 1.24 lb. with a common filter size of 77mm. No weather sealing, but otherwise a well-built lens with fine performance. I have one for low light and fast action shooting when needed. Its price at B&H has been running at $369. Sometimes a fast prime lens will not cover situations where one needs to constantly change the focal length, so a fast zoom lens is the only answer.
