I'm not a huge fan of subreports, but in this case it's probably the easiest approach.
Create a subreport with grouping on the transaction type field. Don't put anything in the detail section. Insert a count on the transaction type, then check the box that says "Show as a percentage of" and select "Grand Total: Count of Transaction Type". Move that summary from the group footer to the group header, then suppress all sections except group header 1.
Go to Report: Group Sort Expert and select Top N where N is 1. The "Based on.." field should already show your summary field.
Place the subreport in the report footer of the main report and link as needed for record selection.