Find out which firmware to use to revert to stock ROM I bought the SG3 (GT-I9300) 6 months ago from Amazon Germany. I have Cyanogenmod 10 installed but wanted to revert it because I'm selling the phone. I want to use this guide to revert it, but I'm not sure which Firmware I should download from sammobile. I didn't get the phone from a specific service provider, so I have no idea which firmware I should use. Any help?
You can use a SamMobile ROM, but be sure that the model, the country and the original carrier match the phone as different carriers often require different radio drivers if they use different radio spectrum for bandwidth.  If you don't know which carrier there are several routes you can use to determine it:


find out which radio you have on your device and match it to the appropriate carrier's 
find out what frequencies your current carrier uses and use that to determine the equivalent carrier for the SamMobile ROMs
the version of CM you installed should also tell you which radio is on the phone, on the CM website
you can also get the information directly by dialing the code *#0011# on your phone, tho it may not be overly helpful (GSM900 is helpful, for example, but WCDMA is not - if it's WCDMA you'll have to look up the Rx(download) and Tx(upload) to determine frequency)


FWIW, I find that Odin generally is easier to use than Kies for flashing ROMs.