I have to vote for modern ignition systems, especially the Tri Spark.
I've done a lot of wrenching and upgrades on my Commandos, but the single upgrade with the biggest influence to overall quality of ownership has been the Tri Spark I put on the 850.
The bike always has started well, but would never idle consistently no matter what I did. Install a Tri Spark and it now idles rock solid at 1,000 RPM.