Any suggestions on a good Video file converter?

If the DVD plays in the DVD Player app on your Mac and/or in a regular DVD player then the video is already MPEG2. UDF is just the file format of the DVD disc, not the video codec. The VIDEO_TS folder contains the files that represent the video. All playable DVD discs have a VIDEO_TS folder.

There isn't enough information provided about this DVD disc or your other 3 discs to figure out why 3 show a preview icon and 1 doesn't. It would appear something is different about that 1.

If you need to convert this to another codec, try using Handbrake.

The first screenshot shows a disc being recognized as a disk (volume), not a video file. This would be normal for a "playable DVD."

The second screenshot shows an .mpg file being recognized as an MPEG-2 video file. What does GetInfo show for the disc media that that file is on?

Assuming these are from two different DVD discs, I wonder if it is the only one that is actually a "playable DVD" and the other 3 are simply DVD media on which video files (.mpg files) were burned. That would explain the difference you are seeing. Do all 4 of the DVDs open in the DVD Player app? Or does the 1 DVD open in DVD Player and the other 3 open in QuickTIme?

You said disc 1 has a VIDEO-TS folder ... in that folder are there files inside called VIDEO_TS.ifo, VIDEO_TX_bup and VIDEO_TS_vob? That would be the normal structure for a "playable DVD." (There may be additional files with names like "VTS_01_1.vob" each xxx_xx_n representing a section or chapter of the full video.) If so, Handbrake should be able to open & convert the video.

This is a "playable DVD" meaning that it will play in any DVD player. When you put it in your Superdrive, the video will open in the DVD Player app and the icon that appears on the desktop will not have a thumbnail image on it. Handbrake should be able to convert this video to other codecs aka formats.

if these will all play, you have not articulated any issues.

what needs to be converted to what?