I'll start by telling you flat out that the best anime of all time doesn't exist since, after all, nothing is perfect, actually it would be really strange if everyone recognized the same thing as the best of all time. I'd totally be freaked out and think that something is fishy. And we all have different lives, we all have lived different things and as such we react differently to the same things.
That question made me think because I have honestly no idea on which criteria I should base my selection. What should I consider first? The entertainment value? The feelings? The plot? The characters? The writing? Animation? Music? Voices?
You can't even separate these, anime is a whole and deciding on a "best" anime of all time is obviously subjective.
Worse of it all? It changes. Everything changes. You'd have asked me a year ago what the best anime of all time was I'd have said : Durarara!!
Because Durarara!! has amazing plot and amazing character development and well, a really, really, really amazing plot, good music, awesome voice actors, very entertaining and not too hard to understand either. Yeah, but.
But something different came out. Not better, just different. Something with easy plot, predictable but not so much it became boring plot, good music, awesome voice actors, awesome writing, awesome character development and feelings. Lots of feelings. Well it's not that much a matter of feeling as a matter of how I identify with said feelings.
More than anything else it gave me hope. And I think that any work that manages to give that to its viewers/readers/whatever completely deserves the title of "best anime of all time" at least in one person's heart.
Now I'm pretty sure you're all wondering what I'm talking about aren't you? If you know me well enough you should have guessed I was talking about Tiger&Bunny. If you don't know me well enough then now is the perfect moment to learn something about me. (But you can forget it if you want ^^)