Zaku, Geara Doga and Kampfer are my fav mobile suits. I reckon they offer a lot of room for creativity.
But if you are looking for the more Gundam side, the ones I can think of are Land Battle Gundam, Land Battle GM, GM Sniper. Considering they are mass produced models that were used in long campaigns, you can go to town on mods and whatnot. Give them a desert paint scheme, jungle paint scheme, improvised weapons and rigs. Anything that's been done IRL since WW2 would most likely work. The sky is your limit
But if you are looking for the more Gundam side, the ones I can think of are Land Battle Gundam, Land Battle GM, GM Sniper. Considering they are mass produced models that were used in long campaigns, you can go to town on mods and whatnot. Give them a desert paint scheme, jungle paint scheme, improvised weapons and rigs. Anything that's been done IRL since WW2 would most likely work. The sky is your limit