Optimal gps/geotagging solution for 5d MK iii

Other than the Canon GP-E2 -what are other options? is that the best available?

Perhaps not the cheapest, but it works well. Since it's both a directly connected GPS and a logger, it can collect data which can tag images taken by other cameras, not having their own GPS, if you use more than one camera at a shoot.

With my 7D, I've also used a Bluetooth GPS puck, together with a Kensington Bluetooth adapter in the WFT-E5B wireless grip. That works too, but is a more expensive solution, if you don't need the Ethernet/WiFi capability of the grip for other purposes.

If you don't feel that you need direct geotagging of the images, when they are taken, but can accept to do that off-line, when you have a GPS data file and the images in a PC, then there are several options, using separate GPS units, that are possible.