Looking for the best generic extruder Robo R1

Not sure what you mean by "generic" depending on which R1 you have (beta/R1/R1+) the ones that will fit as direct replacements (no modifications needed) are different. Most R1 models used the Hexagon so another hexagon (which you can buy from Amazon or other places) will fit. There may be minor differences with the quick-disconnects so you may have to do a little wiring work. There are a lot of other extruders you can use with slight modifications, but those modification vary by which type of printer you actually have.

 

Which printer exactly do you have? Is it the beta? The R1 or the R1+?

It is getting harder to find the generic 12v hexagon Robo used. It may be worth considering investing the effort in converting to an E3D or just get a cheap Chinese clone and hope it works

 

This is the one used for the R1 and R1+ (but again the quick disconnects are not the same)

https://smile.amazon.com/RepRapDiscount-Hexagon-Hotend-Heater-Cartridge/dp/B00W4AICQ8

It may be worth considering the extra steps needed to just convert to something like the more widely used E3D:

https://www.printedsolid.com/products/e3d-hot-end-kit-v6

If you have an actual R1 (not the beta and not the R1+) the instructions are here:

http://community.robo3d.com/index.php?threads/e3d-v6-information-and-installation-guide.17598/

 

The reason we need to know specifically which printer model you have is that while those three models LOOK almost identical they are not

 

Then the generic hexagon will fit fine (the wiring may need connectors inserted or replaced).
The thread I linked on the E3D should be exactly how that is replaced on the R1 if you go that route.