According to Statista the following Twitter accounts have the MOST followers as of May 2012:

 

1. Lady Gaga (24.13 million followers)
2. Justin Bieber (21.82 million followers)
3. Katy Perry (19.52 million followers)
4. Rihanna (18.81 million followers)
5. Britney Spears (16.67 million followers)
6. Shakira (16.02 million followers)
7. barackobama (15.38 million followers)
8. Kim Kardashian (14.64 million followers)
9. Taylor Swift (13.9 million followers)
10. You Tube (12.42 million followers)