We used MP 15 (ruins pack), 31 (war torn) for some of our outdoor levels in our WW2 game. They turned out very nice. A few of the buildings in MP 31 were too modern and were not used, but most of the others fit quite nice. You could also use MP 13, depending on your time frame for WW2 and what your "location" will be. MP11 (outdoor) also was a great help in the design.
Other media from people like Vent, Fuzz, Maxiqueen, Gladius, Doomster, Mgarand and a few others were very useful in our WW2 development. Even Cosmic Prophet's packs were used (landscape- snow and ice from Country Time, etc) and fit perfectly to help create our many outdoor levels.
And of course the Afrika Korps were also a big help.
- BlackFox