This is not easy question to answer as the majority of franchisers don’t provide earnings and potential performance claims in their franchise disclosure documents. And even though well known and respected resources like Entrepreneur magazine publish yearly Top franchise lists they use multiple metrics (including financial data, growth rate, years established, stability, system size, etc) to rank their lists and none that I am aware of are based solely on who is the most profitable.
In my opinion your best bet to determine what franchises are the most profitable is to do your own research on the web, or by speaking with current or former franchisees of franchise concepts you are interested in. I would suggest politely and confidentially asking franchisees if they wouldn’t mind sharing some general opinions about the profitability of their business and its products and services offered. I think you will be pleasantly surprised about how much good info you can discover. Another good sign of high profitability in my opinion is when a franchisee also owns multiple locations.
Also, as with all small businesses, franchise business models that have relatively low overheard, don’t offer a generic or commodity type product, are well established over many years, and have limited competition, generally posses the right ingredients to be very profitable.
Relevant Info: You can explore a number of opportunities that offer high margin services and products here.