I would suggest the best option is the 5.5 inch towed gun or M7, because they were used by US, Canadian and Commonwealth forces and was towed by trucks, which are already in the game as resup.
Russian guns, I have no clue.
For Axis forces, you could say 88mm, pak40, STUGIII, Marder or Wespe, or even 21st Pzer Div gear like Belgian SPG, and you could still be arguing three years later without any mention of Japanese or Italian guns.
I think the late-war 155mm SPG would be a mistake, without the 5.5 inch or M7, besides it wasn't used by any allies except the US, whereas the 5.5 inch and the M7 is going to make a lot more people happy. The M7 also gives you "kangaroo" option.
- 5.5 inch
- 75mm abn howitzer (c47 cargo)
- M7 Priest / M7 Kangaroo
- another SPG
- STUGIII (75mm/105mm options)
- 88mm
- Pak 40
- Argue about Wespe, Hummel, Marder, Belgian SPG.
And same for Japanese/Russian/Italian. That would be the ultimate goal.