Our election process is ridiculously insecure. It is not surprising that a majority of Americans don't trust it to be free from substantial fraud.
209 of the 227 countries for which the ACE Electoral Knowledge Network has data use hand-marked paper ballots. Including France, Canada, Finland, Germany, Ireland, Japan, Netherlands, Norway, Spain, and UK. Because hand marking is less susceptible to machine-completed ballots.
Many of countries (Canada, France, Germany, Israel, Italy, Sweden, UK, etc.) require ID to vote. Because no ID means you can't verify that the person is eligible to vote.
Many of them have one voting day, count the ballots, and have the results the next day. Because less time for the voting process gives less time for manipulation.
The least-vulnerable system is hand-marked paper ballots, cast in person, with ID, on one voting day, and counted that night.
Best system in my opinion would be that, with voting day being a national holiday that makes it easier to vote on that day.