All shots were taken with Canon EOS 70D set on manual mode, ISO 500, fully open aperture. The idea was to compare the details in the image thus the lens sharpness. All images are a 100% crop - 1200 x 1000 px from the original pictures. I chose a subject with a lot of dust particles and imperfections rendering small details.
EF-S 18-55mm @ 55 mm
EF 28-105mm @55 mm
EF-S 55-250mm @55mm
Now, this comparison is not quite fair as both EF-S lenes were tested at their very limits where performance is not usually very hot. Still the 28-105mm is the worst of them all even almost in the middle of the zoom range.
Canon EF-S 18-55mm @ ~26mm
Canon EF 28-105mm @28mm
By far - the worst case!
Canon EF 28-105 in "Macro" mode @105mm as close as it will focus.
Canon EF-S 55-250mm did not have such close focus range so I brought the zoom to 166mm to get similar framing, Again it did outperform the EF 28-105mm
So as it turns out - the lenses, bundled nowadays with the Canon DSLR kits are of a much better quality then what you'll get in yester years and will outperform what was considered "an average" lens 15 years ago.