I shoot with a 7D and have a wide gap between my lenses: the Tokina 11-16 2.8 (primarily for nature landscapes) and the Sigma 120-300 2.8 OS (primarily for wildlife). I would like a zoom to fill the gap.
Weather Sealed - as I find myself consistently shooting in tough conditions: rain, salt spray etc.
Speed - should be a 2.8 considering that I shoot in low light environments often (forest, twilight etc.)
Great IQ - should keep up with the two lenses I already own.
Cost - Spending 2K seems really really excessive.
Canon ef-s 17-55: great IQ and speed, but spending $1000 on a plastic lens seems wrong.
Canon ef 17-40L: great build quality and IQ, but slower than desired.
Canon ef 24-70 2.8L II: $2,300 - ouch
What do you think - are there other options I should consider? Would Canon benefit by improving the build quality of its best ef-s lenses (the 10-22 and 17-55)? I wish they would!
You are not going to find a 17-120-mm f/2.8 lens for any price, you might want to come up with a comprehensive plan to get where you want to be.
Here are some things to think about before you make a decision.
Sometimes we want to go out and buy the best lens for our money and then find out that we did not plan for a complete system, and find ourselves in a quandry, because the missing piece does not exist.
The 7D is not weather sealed at the lens mount where it counts. Putting a weather sealed lens on a non weather sealed camera may lead to disappointment because water will get in at the lens mount.
The 7D is not a low light camera, you will struggle in low light with a f/2.8 lens, even with f/1.4.
APS-C sensors are not the best at wide angles, but excell at telephoto.
I'd recommend that a person plan a system based on their shooting needs, and migrate one piece at a time to it.
Everything is a compromise, with each choice, you will give up something while you also gain something, so give each item a priority and give weight to that in your selection.
Here is my interpretation of the best bodies for some of the uses you mentioned. Its just my opinion. Lens selection is best made in conjunction with a body selection.
Weather Resistant Bodies when used with weather resistant lens
1D X ($7K)
1D MK III (APS-H ~ $2K)
1Ds MK II (FF ~2K)
1D MK IV (APS-H ~3500)
Best bodies for Telephoto
1D MK IV
1D MK III
Best Bodies for Low light
5D MK III
1DS MK III
1Ds MK II
Best Bodies for landscapes
5D MK III
1Ds MK III
5D MK II
1Ds MK II
There is no one camera that is in all 4 lists, so you might look at bodies that are in two or three of the best categories depending on your priorities, and your pocketbook.