Canon EF 24-105mm f/4L IS USM

In 90% of my photos (for stock) I use two lenses : Sigma 50mm F1.4 DG HSM and Canon 100mm f/2.8 USM L, despite the fact that i have other quality lenses.

Personally I will chose a fixed focal lens for stock photography.

With the 60D you can use EF-S lenses.  For travel I use the EF-S 10-22 and the EF-S 17-55.  Both are quite sharp, light weight and not too expensive (but not cheap either).  The 17-55 is also recommended #1 by the Digital Picture for general purpose lenses (http://www.the-digital-picture.com/Canon-Lenses/Canon-General-Purpose-Lens.aspx). 

If you want something really light, small and relatively cheap I would go for the EF-S 24 mm pancake lens and the 40 mm pancake lens.  I have the 40 mm and it is quite sharp but haven't seen the 24 mm yet.  The pancake lenses have the advantage of not attracting much attention when walking around with them and you could easily carry them in a large pocket.  You could get them both for around $300 and those focal lengths would cover a lot of travel photography.

If your main concern is focus rejections then new camera gear may not be the solution - you might need to review your technique as well, and get a tripod too.  And of course nowadays it seems that the right reviewer is the most important - too bad we can't choose them.  Good luck!

I own both the 24 70 f2.8 mk 2 and the 24 105. I also shoot some stock.

If I had to only own one lens it would probably be the 24 105 it's just so much more versatile. If you don't mind spending the money the 24 70 and 70 200 is s great combo.

As other people of said a lack of sharpness is far more likely to be technique. Stock agencies tend to dislike shallow DoF's so from that point of view there is little to gain from the 2.8. For me the big reason for the 2.8 Is lowish light indoor portraiture