I have gotten the most out of the HttpClient toolkit. I used it to create a load tester for my on-line exam servlet. The "codec" collection is pretty handy too - when people write asking for my phonetic encoding (Metaphone) source, I send them to the Codec collection too. Bill [ April 13, 2004: Message edited by: William Brogden ]
I have mainly used the Commons Net project and HttpClient, and once (for prototyping, didn't end up into the production branch) DBCP + Pool. I don't recall using any of the other Commons stuff directly (obviously there are certain dependencies which could be used to argue that I am, in fact, using "Commons Foo" because I'm using "Commons Bar").
I would agree with Pradeep that 'most' useful is relative to the kind of Java development undertaken. For a standard Java web application I would say Digester, HttpClient, Lang and Logging are useful. If you are using Struts, Validator is a must have. The database connection pooling (DBCP) component can be easily integrated with Struts and is useful especially if you are using a app server that does not have DB connection pooling features. If you are writing JDBC code, DbUtils is a simple yet useful tool to have.
Yes, I wanted to know what is really *common* and fits almost all projects, and what is decisive in particular needs. I know every commons component is awesome, so was just kidding the word 'least'. Forget about it please.