There are LOTS of best lenses out there. Unfortunately you did not define what 'best' means to you.
I always, always have my 70-300L with me. If I'm doing something non-specific, I'll also bring the 24-105L. For landscape, any lens will really do. I bought the 14mm specifically for architectural stuff, but I haven't used it as much as I thought I might... I need to go on some small road trips and find places to use it well. If you're doing portraits and street, 24, 35, 50 and 85 are all great focal lengths, and great for fast primes. The 24-70L also covers this range for general use really, really well if you don't mind the weight.
That being said, if you don't have any lenses the 24-105 is pretty hard to beat but the 24-70L is probably the better city street/urban/portrait lens. I don't own the 24-70L, I think it's too heavy and in those focal ranges I'd rather have one fast prime, which I'm currently lacking. lol