If you're not going to build anything complex and don't plan to, Form Master 1.6 (recent update) is the easiest to use. It has a very nice form designer. Stores in xml - rumored to be working on regular sql abilities. Above average support.
If you want a module that is easy to use but can grow with your needs and abilities then Xmod is the choice. It has a built in "auto" form designer for beginners or you can use the custom form designs to do some higher end forms once you have the knowledge/need. Downside is that it stores data in XML (Xmod 5 promises to add the ability to connect to regular sql tables/databases with it's data manager feature). You can do some limited relational stuff with Xmod. Xmod wins for the best overall form module. Xmod also wins in the support, documentation, examples, code samples area. This is also the only one (that I know of) that has add-ons (internal and third party) available for it. Some of those would be a calendar add-on, an extremely nice photo upload add-on, RSS add-on, and a new scheduler add-on that allows you to connect Xmod data with the core DNN scheduler.
Listx/NukeDK wins for the most powerful. You can do even more relational stuff, it stores in regular sql tables, you can connect to different sql tables/databases, etc., etc. There doesn't appear to be much that Listx can't do. Downside is that you need at least average (above average?) programming (especially sql) skills. Pretty good email support, average forum support.
Enterprise Forms wins if you need workflow capabilities. Fairly easy to use and above average support.
dnnkickstart and Module Builder from socanI.org are both very similar. I think Module Builder wins this battle as dnnkickstart doesn't appear to be very well supported (supported = revisions, updates, etc. support = answers to questions, help with problems, etc.). These offer some unique abilites as they are desktop applications rather than modules. Some of the advantages would be that you can build as many modules as you want with them (no single dnn installation limitation). Not having to wait for a ton of post backs is nice too. Module Builder will do some limited relational stuff very similar to what Xmod does. Module Builder is one to watch as they get their "front end" (tags, tokens, etc) farther along. Module Builder has good support not sure about dnnkickstart.
Universal List is also one to watch. Stores in regular sql, will do limited (above limited?) relational data, and now that they have a support forum the support appears to be very good. Module Builder and Universal List are both very good choices for the backend stuff (building tables, adding fields to tables, etc.)
Indoo Grid is probably the choice if you have extremely large amounts of data and just want to show a grid view. Appears to be somewhat limited (?) on the display of data - master/detail, etc. Unsure of support.
Dynamic Forms from Datasprings is geared towards and might be a good choice for training or quiz type applications. Unsure of support quality.
Sigma/H20 I have had experience here but if you can't say anything good............
Combos- from what I understand there have been several people who have used Listx and Xmod in conjunction. Listx for the backend stuff and Xmod to display. One combo that might (??) be fairly powerful in the future could be Xmod 5 (front end / display) and Universal List (back end). Of course you can always use the sql Enterprise Manager for the back end stuff if you have the knowledge and the access to your server.
This is just some of what I have learned so far and is pretty general. Feel free to post corrections, opinions, etc.
Greg