for ultimate comfort, you can't beat a Brooks leather saddle, once it's broken in. they're pricey, but worth it. i've got one on my bike, and it's amazing. i've got the B66S.
for cheap comfort, i have a few of these on other bikes: Pyramid Cruiser Saddle With Springs Black | BikepartsUSA
well built, under 20 bucks, and probably the most comfortable cruiser seat i've found. i rate it right below my Brooks. it's a big fat cruiser seat, where my brooks is shorter and narrower.