I don't think that any of them are in the same level as Streep. Don't get me wrong, sometimes I really feel that Meryl is overrated, but this time I don't think that we can find a person on TV who would be as iconic and as great as Mery Streep herself in the film industry.
People will write Amy Poehler's name, but that's just as silly as their fanaticism. She is good in Parks & Recreation and had some good moments in Satuday Night Live but that's it...
If I have to say a name I'll say Betty White, because she had several roles on TV and she's a legendary funny actress, but once again, she's not Meryl Streep either.
www.actressland.wordpress.com (Best Actress in a Leading Role Ranking Project)
Parker's in the top echelon of TV actresses, but I wouldn't put her as the best outright. If her next project post-Weeds gets her back on the Emmy radar, then you could have a case, especially if she were able to win for it.
For me I feel it has to be someone who is primarily considered a TV actress. Helen Mirren is considered a film actress first and foremost even if she has done notable TV work, whereas someone like Mary Tyler Moore is considered a TV actress first and foremost even though she is an Oscar nominee.
I don't think any TV actor is at the same level as Meryl Streep (although I wouldn't necessarily blame the actor...) However, Betty White and Cloris Leachmen are the two that come the closest. They both have among the most Emmy wins/nominations, and they're still working and getting nominated to this day. And, for the most part, they're extremely well respected. And they've been a part of a multitude of different TV projects and have gotten nominations for lots of them.
I wouldn't say Mary Tyler Moore just because she's faded away and isn't in the spotlight nowadays as much as the other two.
Julia Louise Dreyfuss, and Alfre Woodard could also be contenders. If Tina Fey has a third successful TV project after 30 Rock then I'd put her up there also.
Hugh Laurie once said in a SAG award acceptance speech something about how it really isn't a matter of doing good work but being given the opportunity to do good work and I feel as though Streep has been given more opportunities than anyone else. And that isn't to say she doesn't deserve them but it makes the question a lot harder to answer.