Date Photo taken vs Scaned

There's no ready answer to your question -- it depends on what dates are stored in the photos' metadata. Beware that LR doesn't handle scans missing capture dates very well, with long-standing bugs.

There are three dates shown in the Metadata > EXIF panel. They are defined by the industry standards:

Date Time Original: The date when the camera's shutter was pressed. All digital cameras now set this, but the earliest ones didn't. LR uses this as its "capture date" if it's present.  If it's not present (e.g. a scan), then LR may use for its capture date Date Time Digitized or the file's last-modified date as set by the operating system or camera firmware (the date that any application or camera modified the file, for any reason).  You can change this in LR using the Metadata > Edit Capture Time command.

Date Time Digitized: This is the date when the photo was converted into digital format, e.g. by a scanner. Some scanner programs set this field, but not all.

Date Time: Beware! This is the date when the last photo application modified the photo or its metadata for any reason. It is not the capture or digitized dates.  LR displays this field but doesn't update it (though the standards say it should).

In general, if you want to organize a large archive of photos, it's best to set Date Time Original (e.g. using LR's Edit Capture Time command) to the capture dates of the photos. This will store the capture date in the photos' metadata, making it accessible to other programs. 

The Metadata contains a Folder Date as well as Date time Original.  Is it better to use the folder date to organize by date all jpeg files or........

What do you mean by "Folder Date"?  There is no field in LR's Metadata panel by that name (in the English version of LR).  A screenshot of the entire Metadata panel or whatever you are looking at would help.

You want to use date created I suspect.