I don't know about the best but defiantly of my all time favorites, it made me cry more than once that's for sure and it still gives me feels lol though I suppose that's par for the course on KEY anime I mean Clannad After Story had me weaping like a baby (AS actually is my favorite anime ever lol)
Angel Beats is amazingly impressive for 13 episodes, but I do wish there were more of them so we could get some development for other characters. I also kinda wish the ending went into more detail so that we see what the next lives of the others are like, such as Yuri, Yui and Hinata, and a bit more detail with Kanade and Otonashi and what they live like. Does Otonashi have a sister again for example? Well, at least the main 2 are reunited.