EDIT: Fifth partner found again. We no longer need lumber and already have the following:
-Iron (New Milwaukee)
-Spices (New Milwaukee)
-Pigs (Valitur)
-Wheat (Valitur)
-Cattle (Brd-Arfcomistan)
-Sugar (Brd-Arfcomistan)
-Aluminum (Igbo people)
-Fish (Igbo people)
-Marble (Rage117)
-Water (Rage117)
-Lumber (???)
-*any* (???)
-Bonuses are Fast Food, Construction, and Beer once the circle is complete.
Any second resource not on that list would be appropriate, though lead and coal would provide extra bonus resources (Scholars or Steel). Uranium couldn't hurt either as all of us are or are close to nuke-capable range. PM me in-game if you're interested in never worrying about setting up trades ever again.
List of modifiers:
Initial infrastructure cost: -33%
Infrastructure upkeep: -18%
Population increase: 27.5%
Citizen income increase: $2.00
Population happiness increase: 11.5
Initial land cost: -15%
Purchased land area increase: 8%
Environment: +1
Soldier increase: 35%
Soldier cost decrease: $-3.00
Soldier upkeep cost: $-0.50
Tank upkeep cost: -5%
Aircraft cost decrease: -8%
Aircraft limit increase: 10