K-2SO is the highest profile droid on this list. Outside of perhaps R2-D2, there was no droid more important to the Rebel cause than K-2SO. Saying whatever comes into his circuits, the reprogrammed Imperial Security Droid sacrificed himself in a blaze of glory. That ensured Jyn Erso and Cassian Andor could steal the Death Star plans and transmit them to the Rebels waiting above.
The rest, as they say, is history in a galaxy far, far away.
What other droid heroes do you think played important roles in Star Wars?