Got it. I don't plan on going full-frame anytime soon (but I might be tempted to get the 7D Mk2 if they up the low-light performance) but it's definitely a plus in case I do.
Just to be clear, the 7D series is currently a crop frame, not a full frame solution.
Also, what I am understanding of what you want is a one lens solution that will give you a wide of a coverage as possible while delivering excellent optical performance for as little money as possible.
If this lens existed, every photographer would have it and there would be no other options for people to start off with.
Generally speaking, if you want the best optical performance or letting the most light, you need a prime lens.
A zoom is a compromise lens. It is a generalist. The more it does, the less well it will do any of those functions.
The prime is the specialist. It may not do a lot of things, but what it does, it will do really well.
If you want a zoom (generalist) to do what a prime (specialist) better than what the prime can do, you will generally pay a lot more for that zoom.
The EF 24-70mmL
II is just starting to be delivered to people. It is lighter than the previous model, a little shorter, and delivers optical performance unseen in a zoom in this range before. One of the main surprises in this lens is that it give a higher resolution of images than the best prime lenses. It is also over $2,000 and it is less than a 3x zoom.
The 18-200mm lens you have right now is a great example of a compromise lens. It is great at what it does. The job of that lens is to be able to do a lot of things decently and allow you to have a one lens solution that is not as heavy as a lot of other options. It weighs 470.0 grams.
The EF 24-70mmL
II I mentioned earlier is 805 grams. And people are praising that it weighs less than its predecessor.
So all that said, how about we start by prioritizing what you want.
Please list the following in what order you feel are important.
* Single-lens solution
* Cost (What is your budget?)
* Range (Do you want as wide a range as possible? Do you want to prioritize going closer or further away?)
* Focus speed
* Low light performance
* Optical performance (Sharpness)