The Newstweet Index also pinpoints the top 50 media tweeters, with Sky News digital news producer Neal Mann taking the top spot.
Portland plans to produce the index on a quarterly basis. For the first index, which covers July, August and September, Portland worked with Tweetminster to assess more than 330,000 current affairs tweets from journalists, newspapers and TV news channels.
The Guardian emerged as the top media outlet, with more than 100,000 tweets during the three-month period.
Analysis of the topics covered by each outlet (see below) shows that The Guardian rode to the top of the chart through its prominent coverage of the phone hacking scandal, along with numerous tweets mentioning the police.
The BBC, which came second with 78,000 tweets, gave more prominence to the war in Libya than to phone hacking or the police in its tweets. Making up the top five most prolific media outlets were Sky News, the Telegraph Media Group and The Independent.
Individual media tweeters were ranked according to the number of times they were mentioned or their tweets were retweeted. Mann topped the list, with the Media Guardian Twitter feed in second place and Guardian editor Alan Rusbridger in third.
Journalists linked to The Guardian and the BBC dominated the top ten, but Channel 4 News economics editor Faisal Islam took 11th place. The highest placed FT journalist was education correspondent Chris Cook, in 18th, while the highest placed Independent journalist was foreign editor Archie Bland, in 20th.
For the Telegraph, the best performing journalist was blogs editor Damian Thompson, in 25th. For The Times, political correspondent Michael Savage claimed the highest ranking, coming in at 35th.
No journalists from the Daily Mail, The Sun, Daily Mirror, Daily Express or Daily Star featured in the top 50 media tweeters.
Commenting on the index, Portland MD Tim Allan said: 'Twitter is fundamentally changing the way that stories break, spread and develop. We have developed this index in order to give the first proper insight into who the movers and shakers are among media tweeters, what they are discussing and how they interact.'
THE TOP TEN MEDIA TWEETERS
1. Neal Mann, digital news editor, Sky News; @fieldproducer
2. MediaGuardian.co.uk; @mediaguardian
3. Alan Rusbridger, editor-in-chief, The Guardian; @arusbridger
4. Andrew Neil, presenter, This Week and Daily Politics, BBC; @afneil
5. Guardian News Feed; @guardiannews
6. Ben Goldacre, author of the Bad Science column, The Guardian; @bengoldacre
7. BBC Radio 5 Live; @bbc5live
8. Charles Arthur, technology editor, The Guardian; @charlesarthur
9. Paul Mason, economics editor, Newsnight, BBC; @paulmasonnews
10. Grace Dent, columnist, The Guardian; @gracedent.
... and here's what they are talking about
Total tweets from media organisations*, and the most popular topics