California's system strikes me as ideal. Ballots are on paper but are machine-countable. Voters feed the ballots into the counting machines themselves, which verify that the ballot is valid (e.g., voted for exactly one person per race). If your ballot gets counted properly, the machine makes a happy little noise.
I like this because it has the fast results of electronic voting, but it has a proper paper trail and minimum mystery about the counting process.
What system, in your opinion, would minimize error?
What on earth? I marked the candidate I wanted to vote for with an X in a circle on a paper ballot. Marking circles for more than one candidate is a spoiled ballot you can replace instead of depositing if you wish. This isn't rocket science.
There's still plenty of margin for error (for instance, the printer could run misprint or run out of ink, the tabulator could be buggy), and corruption ("here, let me file that for you...") but I think it maximizes the benefits of touchscreen voting for disabled voters while minimizing the downsides.