ww1 all they way, second world war has been covered so many times that its just too redundant. need a fresh scene in the fps community, ww1 would be the most interesting for me, however id say that the korean war would be a close second.
AS far as ww1 goes there is much people dont know about it, id also like to point out that there was indeed a eastern front, there was a front around the mediteranean, a front in the middle east, and also an Italian front.
The italian front could be the most interesting as it saw trench warefare up in the alps, i mean cmon who doesnt want to fight up in the mountains, or the massive movements of the ww1 eastern front, or the gritty trench warefare of the west, or the open scaling of the middle east or battles like gallipoli, i mean the possibliities are endless.