Best or cheapest? The two are not the same. You said best, then you said cost consideration. So I am confused.
It depends on the application, size and use of the roof. It depends on who is installing it.
There are numerous flat roofing options. What is "best" depends on the use of the roof and design. There are seldom ever any one size fits all roofing solutions. Learn more about your flat roofing options here: http://reliableamerican.us/services/...e-roofing.html
A self adhering modified bitumen roof is fairly DIY friendly and if the roof ties into a shingle roof I sometimes prefer the granulated modified bitumen roofing system. A self adhering modified bitumen roof will require more upfront investment than some other roofing systems. EPDM is also another DIY friendly roofing system. Though EPDM is single ply and is less forgiving if you make any mistakes. Where as the self adhering modified bitumen is usually at least 2 layers, preferably 3, so chances for failure are minimized. I usually recommend people not try doing DIY roofing if they don't have a good understanding of construction methods. A repair? Sure, but a roof replacement if done wrong, can cause thousands of dollars worth of damage.
If you are hiring a roofing contractor to do this, that really opens up tons of varieties. I personally prefer thermoplastic membranes sucha s PVC and TPO. I can really TRUST the heat welded seams. My guys really like EPDM, and so do I, for the ease of installation, but the adhesives at the seams and penetrations are known to have issues over time.
If you choose to do this yourself, read the installation instructions and perhaps the manufacturer will have some videos. The base sheet can be nailed or screwed down. Then you would install the optional adhesive mid ply, or the mid ply can be used as a base sheet and stuck down directly to the wood. Finally the granulated cap sheet installed also with adhesive. I don't like self adhering modified bitumen roofs on really dead level roofs. I always want a good slope with this system. So far so good.