Not from Iowa, but across the Missouri in Nebraska.
My best luck has been with the hybrids - Grand Duke hybrid has done well for me in the past, last season I tried Kolibri (purple) and Kongo (white), most of my plants succumbed to the tragedy of my Bt spray (I misread the amount of wetting agent to use and burned all my brassicas) but the few that missed the spray seemed to do good. Trying both of them again this year.
Early white/purple Vienna get way too woody for me. Haven't tried the large bulbed (Superschmeltz) types.

I've had excellent results with every type that I've planted, both in the spring and fall plantings. As Cybrczch says, Some of the early varieties tend to get woody if you let them get too big, but it usually isn't a problem for me. They get eaten quickly!

Mikkle, curious as to how your kohlrabi did this year. I started mine indoors to transplant, I planted 18 Kolibri and 18 Kongo plants out in mid-late April, harvested the first Kolibri at the end of May and picked the last 3 Kongo 2 days ago. Big, but not woody, could eat all the way down to the stem area. This year I also tried roasting them, cubed, tossed with oil and salt, then top with parmesan. Not bad at all.

I have had good luck with Kossak hybrid direct seeded in the spring produce baseball to softball size tender and sweet