There is some misinformation floating around here. The >=11 speed Ultegra and Dura Ace cranksets are glued together with epoxy, which sometimes becomes unglued. The Shimano 105 crankset is all aluminum with no glue joints, and do not have that particular problem. So the 105 crankset is the best choice for a 160mm crankset.
If you want to go below 160mm, you can try the Origin8 square taper cranksets on Ebay. . I have used both the 110mm BCD type and the 130mm BCD type and they both worked fine on my kids and wife's bikes. I also tried the BBK crankset but that one uses the problematic GXP BB and I do not recommend that one. Cobb is another choice but that one also uses GXP BB.