Flat roofs are often viewed as big detriment to a property because they draw so much heat into a building and seem to be almost impossible to fix. But newer materials can keep a flat roof worry free for 25-50 years. IMO the best option for a flat roof is TPO roofing. Instead of typing everything out, I will post a link from a TPO manufacturer. The energy savings are excellent, and the durability is unsurpassed(to my knowledge). Publix grocery stores are using TPO on their newer stores because of its efficiency. Two people can carry a 1000 square foot roll, so it's a good DIY material for small roofs, but I would recommend hiring an experienced contractor for jobs big enough to require seams. Here is the link, don't be scared of a flat roof! GAF brand TPO vs Rubber roofing
Too many factors to promote a one-size fits all solution.
I've got 22 years on a HDM(?) rubber roof, still works, no complaints, over wood decking, glued, torched and screwed. There might be something better, but I don't need it yet. :)