i use the curl function in php via command line hitting the mobile version of the site.. if your going to do a lot, you will need proxies once you reach the api limit... im not exactly sure how the api limiting works but i can load about a page of followers per second for about an hour, then the proxy will be blocked for about an hour
a side note, even tho the way i write my curl scripts is made to look like a normal computer user and not a bot, it still hits the api limit. just in case you were wondering
also when your loading thousands or possibly millions of follower pages, you will hit your fair share of fail whales, timeouts, and other errors so you if you want to grab all of 15million followers for a popular account you will need to program in some error handling