There are lot of "What lens should I get?" threads here, and rather than answer them individually, I thought I'd list the three lenses I always have in my bag, and why. I hope other photographers will jump in and list their favorites as well, so those looking for lens recommendations will have a concise thread to read first.
I shoot with a 5D Mark II, so my preferences are based on using a full-frame sensor, although I'd used similar lenses with my 30D. Listed by preference:
1. 70-200mm f/2.8L IS Hands down my favorite lens. It's fast, incredibly versatile, and consistently nets me more keepers than any other lens I own. Whether I'm shooting my son's baseball game, a Craftsman Bungalow, or a neighborhood rose garden, this is the first lens I grab. The only drawback to the lens is its weight and size. The non "IS" version is a bit lighter and less expensive, but the added flexibly of a 2.8 lens with three full stops of IS makes it a no brainer. Buy this lens, you'll love it!
2. 16-35mm f/2.8L This lens is simply fun. Put it on your camera and you start seeing things in a whole new way. This is my lens of choice for shooting showrooms and landscapes. I'll put it on my camera as I'm walking around and just look at things from odd angles. The lens constantly surprises me. The images are, of course, beautiful, with great bokeh and remarkable edge to edge sharpness even wide open. Keep in mind this lens was really made for a full-frame camera, so if you're using a 30/40/50D or the like, it won't give you the same experience. 25-56mm just isn't the same thing. That's a useful range for sure, but no longer what I'd call "wide angle."
3. 24-105 f/4L IS We've arrived at the difficult, and somewhat,controversial,choice. Yes, the 24-70mm f/2.8 looms large here. There's a lot to recommend about the 24-70, but I'll tell you why I prefer the 24-105 in my bag.
This is what I consider my "walk-about" lens. A "jack of all trades, master of none" lens if you will. If I don't know what I'll be shooting, or what I'll encounter, but want to have my camera around my neck ready for anything, I put this lens on the 5D. In this regard it has a huge advantage over the 24-70 in that it's significantly lighter. The weight is also a consideration in a bag with the 70-200mm f/2.8L IS lens. I've got a bad back, and anything that reduces the load is welcome.
Do I miss 2.8? For sure, especially when it comes to motion in low light. However, the IS on the 24-105 gives me three full stops of stabilization, and I can hand hold 1/3 sec and still get tack sharp shots. A little longer even if I haven't had caffeine. [] That's remarkable really, and for a walkabout lens, meaning you rarely have a tripod, it's incredibly useful. And, as Bryan's review will attest, the 24-105 is no slouch when it comes to image quality. In fact, I use it for catalog product shots all the time, and find the sharpness and color to be outstanding.
Finally, the additional reach of the lens is something you'll appreciate far more often than you'd think. Without the overlap with my 70-200mm, I'd have to do far more frequent lens changes, something that isn't fun and drains the creative juices when you're shooting. There's no getting around it, 24-105 is a far more useful range than 24-70.
The 2.8 is really, really tempting, and there are times I wish I had it instead, but more often than not I'm happier with the 24-105mm f/4L IS.
FWIW, I also keep a 2x converter in my bag. It's light and easy to carry, and is great for those times when I really need the extra reach. The 1.4 is probably more practical with only 1 stop lost, but when I want reach, I really want reach!
I hope that helps some lens shoppers. I hope others jump in, as I love reading about others experiences and choices. I always learn something.