According to recent analysis 123456 is the most commonly used password[1], however; these passwords all appear on Twitters list of 370 banned passwords[2] and should be avoided at all costs! Common passwords are amongst the first passwords to be cracked when under attack.
[1] https://www.telegraph.co.uk/technology/news/7042205/Top-ten-internet-passwords-to-be-avoided.html
[2] https://www.businessinsider.com/twitters-list-of-370-banned-passwords-2009-12