That's a big question. I'll try and answer it briefly.
Foam Board would be the cheapest for sure. It doesn't require much special equipment and it's durable and lightweight. Here's an example of the B-52's predecessor, the B-36, done in Foamboard. It has a 6" wingspan and is a very advanced build, but you could use the plans for inspiration.
https://forum.flitetest.com/index.php?threads/6-wingspan-b-36-w-edfs-andbombs.53961/
Hotwire foam would be another option. This is where you take foam insulation and cut it with a hot wire. Here you need more expensive materials but you can get very detailed models that look incredible. Here's a build thread of someone who did a foam B-52 with a 7' wingspan.
https://www.rcgroups.com/forums/showthread.php?2880197-B-52-7-ft-SPAN-Foam-construction
Balsa is also an option, but that is the most expensive and time consuming way to build it. I don't have any examples for you, but I will suggest you don't begin your scratch building journey here.
So, in summary, foamboard may not be the best option for building a B-52, but I believe it is the simplest option and can yield some very good results.