Meryl Streep without a doubt
Let me explain why the other top contenders are not at the level
Foster.... she hasnt had stability, shes had some great roles sure but for each one great role shes had I can name 3 or 4 that Meryl had within the same period. Also range of characters, nah shes just not at the same level as an actress (yes shes a director too but thats not what we are judging here)
Bullock.... I actually think Bullock has more range than Foster but still Bullock has just earned respect as a drama actress since she won the Oscar (great role sure) but before that she was a B movie star and she was hired mostly for romantic comedies and not great roles
Roberts, Jolie, Weaver.... sure theyve done great, but sorry they dont have the name and respect that Meryl has as an actress, if you dont see that you probably dont know much about movies, thats all