None of these worked for me. I think i already had mysql somewhere on my computer so a password was set there or something. After spending hours trying every solution out there this is what worked for me:
$ brew services stop mysql
$ pkill mysqld
$ rm -rf /usr/local/var/mysql/ # NOTE: this will delete your existing database!!!
$ brew postinstall mysql
$ brew services restart mysql
$ mysql -uroot
all credit to @Ghrua