OK.....somebody mentioned my name ;)
Siggy 18 - 50 EX Macro 2.8. As Eileen says one stunning lens. I use a mid range zoom a lot and could justify buying any of them if I needed it. For the last two years I've used the Sigma. Very sharp, nice contrast, small and fast. It's also cheap.
Nikon 17 - 55 2.8. I've used this lens a little. It has a very strong reputation as Nikon's classic medium zoom. I haven't used it enough to know but the shots I've seen from it don't make me rate it much higher than the Sigma optically. Build quality is something else - if you abuse your kit then get this one because you will break the Sigma (I did....).
Nikon 24 - 70 2.8. Almost certainly the best zoom Nikon have ever made. Well worth the grand I paid for mine
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. Outrageously sharp and loads of contrast. This has been called the new standard to measure all lenses by.
But a couple of other thoughts.....
1. All of this is based on real world use - I don't test lenses in labs or pay much attention to test charts.
2. The 17 - 55 and 18 - 50 are both DX lenses - great on crop sensors, not good on full frame.
3. Most people seem to like the
effective 24-70 length which is roughly what a 17 - 50 is on a crop camera. You may find the 24 - 70 too long on a D300.
4. I've never used a D300 - it may be that the 24 - 70 has a magical relationship with the D3. Seriously.
For a crop camera I'd get the Sigma (yes, instead of the Nikon 17 - 55). For a full frame the 24 - 70.
Kidography. It's like photography. But more fun.