Which of these Alsace wine region towns should I visit in one day?

We stayed in Ribeauvillé and it has one of the larger medieval centers and 3 ruined castles above the town. It also has the largest wine cooperative in the region, dating back to 1895. You can tour medieval Ribeauville and then taste a Riesling or two while waiting for your return bus which will stop
near the cooperative. Riquewihr is amazing and unique because of its castle, medieval gateways and fortifications. It also was home to the person who, centuries ago, was responsible for the term “Gourmet.”

Pope Leo IX was born in Eguisheim when it was actually part of Germany. The church and town are crazy adorable. We stayed in the town at a working winery. https://frenchmoments.eu/eguisheim/ Tasting is available whether you stay with them or not.

https://www.tourisme-eguisheim-rouffach.com/EN/Domaine-GINGLINGER-Paul-f253001030.html

It's a difficult choice to make. Since you only have time for 2 villages, I would eliminate Hunawihr. I don't think there's enough to keep you occupied until the next bus.

Ribeauville, Kenko has given you a summary. It is the largest of the villages.

Riquewihr, as Kenko pointed out, has medieval gates and fortifications, with its winding streets giving it a fairytale feel.

Kaysersberg, lovely village on a river, the birthplace of Albert Schweitzer

Eguisheim is smaller than those above, Jules' description of crazy adorable is perfect.

Turckheim is the least touristy of the 5 villages with a nice little WWII museum on the Colmar Pocket.

All the villages have their own charms and feel and deserve some time.

Before catching your train back to Strasbourg, visit the Little Venice area in Colmar's old town.

Last year in early June, we used the Kutzig bus and stopped at Riquewihr and Kaysersberg. Riquewihr was mobbed so we left as soon as we could, so keep in mind it might be very crowded. Perhaps others will comment on their recent experiences. If less crowded, I’m sure we would have liked it. The village of Kaysersberg was lovely. We could have visited a third village, but chose not to.

We found Riquewihr delightful (same for all the others), We were in Riquewihr by about 8:30am. Better for finding parking and the crowds (and I mean CROWDS) didn't show up until about 11. I'm guessing some of that was tours arriving.

Do you enjoy walking/hiking? If so, you might like to follow our itinerary. We took the bus from Colmar to Ribeauville, which is very picturesque. From there we hiked up to three ruined castles, which was a lot of fun. Then we walked through the vineyards to Riquewihr, another appealing village. From there, we bussed back to Colmar. It was a very enjoyable day. We were there in March, so there were no crowds at all. Sorry, I do not remember where we ate lunch.

Thanks for all the input! Sounds like Riquewhir might be best to head to first before it gets to crowded. We do enjoy hiking, so I'll look into that option!