In a most general sense, it would be Modbus -RTU on RS-485 if in case I am right in assuming that you are making a instrument for the industrial sector. Across most countries in the world, most small and big manufacturers of electronic instruments provide this. It is an open protocol and easily implemented into your microcontroller. RS-485 has the advantage of being multi-drop, in addition to being compatible to RS-422 devices. If you are targeting PLCs
as masters for your product, Modbus on RS-485.
These days, there is increasing use of Modbus over the ethernet physical layer.In case the data volume is quite less than a few hundreds of memory bytes, you don't need ethernet really Ethernet to RS-485 controllers such as the ones made by MOXA are quite inexpensive. Please lookup Modbus.org
You could add a DeviceNet flavour to your product, if you are in the Allen-Bradley ecosystem where your products intermingle with theirs on the shop-floor. Same idea goes for the Profibus /profisafe/Profinet protocol& Siemens ecosystem. For DeviceNet licensing lookup ODVA.org
If your product wants to communicate only 'bits' of data it is the ASI protocol. This is quite common in the machine-tool world.Ref. ASI.org
If you are selling to the DCS world, try and incorporate HART. But this is a far more sophisticated exercise
If you are making a lab instrument, merely to display data/few pages on your own Visual Basic/VC++ application then pure and simple ASCII on RS-232/USB will suffice. To implement this, ref the book 'Serial port complete' by Jan Axelson
Last but not the least, must not fail to mention the CANopen protocol. It has terrific speed plus a number of advanced features. It is also used by small-scale device manufacturers even in India.
For wireless links use Zigbee!
there is no end to this subject. I hope I've given you enough leads to get started.