There is no such thing as the "best" anime of all time, since it would be impossible for everyone to agree on 1 single anime to be the best of them all.
You could ask what most people have as their personal favorite anime of all time, and that would be a different story.
As for me, my #1 favorite anime is Cowboy Bebop. But, overall, I really like it just as much as my #2 favorite anime. So, usually, when I list down my favorites, I just say that my top 2 favorite anime are Cowboy Bebop and Yu Yu Hakusho.
I'm sure that plenty of people would agree with me on Cowboy Bebop, if I said that it was the best, but I can guarentee that almost nobody would agree with me on YYH.
The point is, though, that I couldn't call either of my favorites the best, since there would also be plenty of people who would disagree with me, on what I think is the best.