Some good zoom lenses:
This is a Canon 24mm – 70mm f/2.8 lens that will carry the f/2.8 speed throughout all the variable focal lengths.
Here’s another zoom lens that will also carry the lens’ speed throughout the zooming range.
Here’s one that will open up to f/4 when it’s used on a wide angle setting, but only open up to f/5.6 when used on its telephoto setting.
So, the best advice I can give when buying any lens is to buy the fastest lens (lowest f/stop number) you can afford. Obviously, a lens that has an aperture of f/2.8 will be more expensive than one with an aperture of f/4.