I guess it depends on what your cost range is, what camera you use, and what your goals are. I myself am a canon user, so I can give you options that would serve a canon camera particularly well. I can't say that I can help much for other brands.

If you need a cheaper, middle-grade, middle-range lens that will cover most common situations, a lens in the range of 20-130mm will probably suffice. Canon makes two lenses in this range, of different grades: the EF 24-105mm f/4 IS USM L, and the EF 28-135 f/3.5-5.6 IS USM.

The 24-105 lens is a higher end L-series pro lens. Its an excellent quality lens and a good performer. It has an f/4 aperture, which while ok for general lighting, will not perform as well in more fringe lighting scenarios like indoor sports, morning/evening, or night shooting. The 28-135 lens is very similar, but is one of Canon's middle-grade lines. Its quality is not quite as good as an L-series. It does offer a slightly larger aperture at f/3.5, but it is not constant, and closes down to f/5.6 as you zoom. It does offer slightly greater zoom range, however, which may be useful.

If you are looking for a single lens that is truly multi-purpose, and will cover any situation you may find yourself in, then there is also a large zoom range lens for that as well. The EF 28-300mm f/3.5-5.6 IS USM L lens is one of the ultimate multi-purpose lenses I've ever encountered. It has one of the largest zoom ranges in existence at 11x zoom. This lens, at the short end of 28mm, is not particularly wide with an f/3.5 aperture. However, as it extends towars the 300mm long end, an f/5.6 aperture is not too bad. The quality of this lens is pretty good throughout its range, although it certainly is not as good as a 300mm or 28mm prime. Given that it is a single lens, it is highly adaptable for a broad range of situations, allowing you to take one lens and leave the rest at home.

Canon lenses usually come with a price premium, and there are some other brands that offer similar lenses at similar quality, and cheaper cost. You might also look into the following:

• Sigma 50-150mm f/2.8 II EX DC HSM Lens
• Sigma 18-250mm f/3.5-6.3 DC OS HSM Lens
• Tamron 18-270mm f/3.5-6.3 Di II VC LD (largest zoom range period at 15x)