For me - Outlook 2003 - especially since I can connect to my work Exchange server via HTTP, and I can access Hotmail through it. Also, I sync my PocketPC with it - which includes syncing contacts, calendar items, tasks and notes in addition to email.
In addition, I use the following plug-ins: SpamNet for spam, NewsGator for RSS, LookOut for searching, and 'Getting Things Done' for personal productivity.