No such thing… it depends on the exact location and the equipment available to you. Can even differ from floor to floor.
If on a low floor, NT Wireless Thunder 3 month package could potentially be the best, but you need to have equipment that can see the signal. Next potentially best option could be DTAC unlimited 30 Mbps, again location and equipment permitting.
Again, due to location, AIS 20 Mbps could be better than a 30Mbps connection that doesn’t deliver it full potential due to location. https://www.lazada.co.th/products/ais-20mbps-ais-super-wifi-i3834334596-s16374244388.html
The only way to know is to do speed tests with a network SIM at the location. If it delivers then one can obtain a SIM package as described, using smartphone or modem as aforementioned.
Too simplistic to write ‘unlimited prepaid’ as a solution.