If time only permits a visit to 1 or 2 out of these 3 towns, which would you choose and why? How many days do I need in each to get a nice visit without being too rushed? Thank you!

Any of these 3 towns can be visited in half a day, but the real gems are in the respective hinterlands.
All depends on your interests and on your itinerary.

Baden-Baden could be a destination itself - tour ornate historic casino or wager there - we spent a delightful day there -especially nice for walking in a park-like setting but yes these towns themselves have little more of interest than a day or so but are great bases - what are your overall trip plans and how much time do you have for these places?

We have 10 days/9 nights total, need to spend at least 2 nights in Heidelberg, flying in and out of FRA. Can't hit all these towns in the other 7 days plus see a bit of Munich so trying to narrow down to 1 or 2 of the towns I list in the OP. Thx!

Freiburg would make a better base for drives thru Black Forest and IMO is a much nicer town all in all.

I liked the colorful SMALL town of Colmar. So walkable and typically Alsatian. You can meander down twisted cobble alleyways at a nice slow pace!

Add Petite Venise, the Unterlinden museum and a crepe....heaven!

I liked Colmar; and I loved Strasbourg, which is also very nice and not far away.

Konstance is also beautiful, especially in spring when you can combine it with a visit to Mainau, the Island of the Flowers.

We enjoyed Colmar. Strasbourg was nice. The little villages of Riquewihr and Eguisheim glorious (very close to Colmar).

Colmar, Strasbourg and Baden Baden are very nice.

But I second the idea of visiting the Unterlinden museum in Colmar. Musée Unterlinden - Colmar | Any Port in a Storm