I use
http://twitaddict.com/hummingbird/ (paid tool)
No idea if it's the best, but it does the trick. It opens in its own browser, so it simulates a human doing the work.
You can get banned going crazy on it.. but I do it like this..
Setup and hand follow about 50 people, takes like 2 minutes.
Then setup with
http://www.twollo.com/ (free tool) to get auto following going.
I then use several tools to fake my posts, but mainly: hootsuite.com (free and it tracks clicks of links sent through the service, so great tool)
And now.. Wait two days, and jump it to 250, next day 500.. Then I wait 2 days and clear non-followers. Then I bump it up and repeat in blocks of 250-500ish.
The bitch is you can't follow more than 2k until you have more than 2k following.
So when you get close, start posting more text than links, like 10 to 1. Then go follow people you actually like, are active, that follow people back (equal following/followers)
The last 250 to 2000k is the slowest part. Once over 2k, it's a 10% follow rule. So it's easy for you to see how fast you can grow.
Every group of following roughly half follow back. So you go follow like 185 (gota leave room for auto follows), and roughly 1/2 will follow back. You can start doing this daily, but I rotate through the accounts personally giving each plenty of time to do some natural building.
Don't forget I get 10% of what you earn.