Iowa (updated January 17, 2020)

The best machines at Horseshoe Council Bluffs are three 100-play machines 25c machines. These are only 98.98% return, and the 100-play brings out some pretty high variance, so you are best off finding another option if possible.

9-6 Double Double Bonus 25c/credit, 100-play ($12.50-$125 per hand), (98.98% return). Located in the middle of the casino, about 50 yards due north of the players club.
Average loss getting to Diamond: $510
Average loss getting to Seven Stars: $5100

25-15-9-4-4 Deuces Wild ("LV Airport Deuces") $1/credit 1-play (98.91% return). Will take you 10 hours or so to earn 2500 tier credits -- maybe more if you're not that familiar with Deuces. Not a very good option. These are bartop machines near Caesars rewards.
Average loss getting to Diamond: $545
Average loss getting to Seven Stars: $5450



The same deuces game is also at Harrah's Council Bluffs.

25-15-9-4-4 Deuces Wild ("LV Airport Deuces") $1/credit 1-play (98.91% return). Will take you 10 hours or so to earn 2500 tier credits -- maybe more if you're not that familiar with Deuces. Not a very good option. These are at the TAG bar, just inside the casino entrance.
Average loss getting to Diamond: $545
Average loss getting to Seven Stars: $5450