I can't remember, maybe I used to use rankings when I was a newer fan, but nowadays I primarily look at what appeals to me. The problem with ANN's top rankings that that newly-airing series tend to shoot straight to the top and get overrated. This happened with Haruhi, Death Note, Code Geass, Clannad, and probably some others. Then once a wider audience gets exposed to them, they tend to fall dramatically.
But generally, the rankings at ANN/AniDB/AnimeNFO/MAL are a good starting place if you want to find "anime that lots of people consider good."