Canon 17-55mm EF-S IS USM Lens
The Canon EF-S 17-55mm f/2.8 IS is one of the first EF-S lenses Canon released and its performance suggests Canon is very serious about supporting the EF-S mount with professional level lenses. Though not a proper member of Canon's professional "L" lens line, the EF-S 17-55mm f/2.8 IS is nevertheless considered by many to be Canon's only EF-S L lens, if just de facto.
The Canon EF-S 17-55mm f/2.8 IS has a large 77mm front thread to filters, and that's just one clue that this lens means business. Typically, Canon's professional lenses have large front threads because the front element on high performing lenses tend to be large, so it's the nature of the beast to require large filters. One criticism of this lens, and one used to show how the EF-S 17-55mm f/2.8 IS wasn't "professional" grade, was that the lens body is a rigid plastic polycarbonate. Not only is polycarb a perfectly capable material, many of Canon’s L lenses are now also made of polycarbonate!
This lens is made with both Ultra-low Dispersion (UD) and aspherical optical elements, so rest assured the features that most directly contribute to image quality are there. In addition, the Canon EF-S 17-55mm f/2.8 IS (denoted by the "IS") has built-in image stabilization, a rare but very useful feature among fast f/2.8 walk-around lenses.
Though there’s much controversy as to the value of UV protectors on modern lenses, there exists many anecdotal stories of the Canon EF-S 17-55mm f/2.8 IS benefiting from a UV filter being mounted. There may be enough evidence to suggest a UV filter prevents or inhibits dust from entering the body. Actually, if you look closely at the front of this lens, there appears to be a small hole right out the front element in the bezel that may be the culprit. Blocking this hole, which may be for pressure release as the body extends and contracts for zooming, might help stop dust from entering the body, possibly requiring cleaning. So, no matter which side of the UV protector debate you fall on, you might want to consider one where this lens is concerned.
If you shoot on a Canon crop camera, like the Rebel XSi or the 70D, among many others, consider the ultra-sharp, fast aperture, and quick auto-focusing Canon EF-S 17-55mm f/2.8 IS. It might be as close to an "L" lens we'll get on an EF-S mount lens.