This is not an easy question to answer. To determine what printers should be used as a “standard” one needs to know the number of pages that would be printed each month, the number of people with access to the printer, the type of printing that is needed (letter, legal, transparencies, labels, etc.) and whether one needs color or nat as well as whether duplexing is needed. Then you have the MFP (multiple function printers) and the inherent questions about what functions are needed, scanning, FAXing, etc. Then you have the questions of all printers, whether you should use ink technology, laser technology or wax (some Xerox Phasers and others use this–relatively rare in my circle).
Once you have sorted all this out, then can you start looking at printers to fulfill your needs, and develop a standard list of printers for your company. Unfortunately, the pace with which printer manufacturers are now introducing new printers and obsoleting older models would mean that you probably would need to research the market every six months or so.