Failed logins are really the most important. It just depends on how busy your system is. It is all preference on what you would like to monitor.

I use the maintenace database to hold procedures and tables for...

reindexing, capturing database size for trending, capturing critical events from the error log, etc.

Jonathan Kehayias did a real good presentation called Automating Common DBA Tasks that has a lot of other good ideas. It can be found under the live meeting recordings here.

http://www.sqlpass.org/Community/SIGs/DatabaseAdministrationSIG/tabid/80/Default.aspx