Like Rob, I too have over a 1/4 century of Service, so there's a LOT to choose from.. BUT...
My primary is going to be the 1/35th Italeri M109A2 (converted to an A3, which is just a refit of the A1 to A2, IIRC) and an AFV 1/35th M548 Cargo Carrier, with assorted Tamiya, Dragon, Testor's and a Soviet-made set (who's name escapes me at the moment. IRC or IPC or something like that..), since i "Frankenstein" figures... Both kits will be for a diorama of course, but I'm gonna call them finished when they're ready to mount on the dio if that's OK...
The reason I ask is that there's always SOMEthing to do on the model after it's mounted, even if it's just a little pastel-dusting to tie the model to the terrain.. Also, figure placement will be after the fact as well..
My back-up, should the howitzer fall through (parts missing, broken, etc), or if I have tie to do two builds, will be "my" trusty little M151A2 that I drove through hell & high water at Ft Hood during one FTX, where I was double-slotted with another guy as NBC NCO and I drove the BC instead... There's a story about me, the BC, the quarter-ton, and an old foxhole at night that's pretty funny... NOW...