They are no best films in which to invest as it is whatever proves to be a commercial success. Traditionally horror films show a greater return on investment as they tend to be lower budget productions, and have a captive, albeit limited audience.
The potential success of any film relies on a number of factors, including script, cast and then the critical reviews and marketing spend by the distributors. However there is no guarantee that any film will be commercially successful.
Ideally an investor should invest in a slate of features covering different genres and budget levels, to maximise the prospects of a healthy return.