Finding the NEM Kadee with the best fit will vary greatly depending on each piece of stock and how tight the layout's curves are. #19 Long Kadees are a fairly safe option on locos and wagons; it is also a safe bet to use #18 Mediums Kadees on typical 4-wheel wagons. #20 Extra-Long Kadees are only a good choice on stock where the NEM pocket is far back.
Of important note: quite a few Heljan and Bachmann items do not have the NEM pocket at the correct height: a method to get the Kadee at the correct height must be used (e.g. moving the NEM pocket down, gluing the NEM Kadee to the und