Personally, in terms of movie actresses similar in age, I greatly prefer Glenn Close, Sissy Spacek, the aforementioned Diane Wiest, Sally Field (when she's inspired like in Lincoln), and Mary McDonnell. However, I think it's a fool's errand to try to convince people Meryl Streep is a bad actress. She's not. She may not be your cup of tea and you may disagree with her interpretation of the characters.
IMO, the best Streep performance post-1980s was her role in Adaptation. She seemed less Streep by actually tearing down the pretentious and just played the character straight. When she has a good director, the director knows how to keep Streep from over-doing her mannerisms. When she is given more free reign, her ticks become unbearable (shifty eyes, playing with her hands, etc.).
That said, she is great and I'm happy to see an older actress can still be a global box office draw. However, her being the "one" doesn't really help the cause much, does it?
"I've got to get me out of here
This place is full of dirty old men
And the navigators and their mappy maps
And moldy heads and pissing on sugar cubes
While you stare at your books."