My five would be:
Pocket Informant for all calendar, contacts, notes, and tasks
http://www.pocketinformant.com
Gigabar can launch everything and close anything from any screen
http://www.gigabar.com
Mad Journal for daily journal type notes. Opens to current day.
not sure if it is still available but you may try
http://www.pocketgear.com
Flex Wallet for all the money, passwords, and other sensitive records I need on my Ax. The bundle with the desktop edition and the Pocket PC edition is a great set that covers it all for me.
Bug Me for all the immediate things that come up that made paper so hard to get rid of . . . With rich text, colors, and a good interface, it is the fastest way to take notes quickly and set alarms easily in a colorful and even artistic way. Its almost like a "MS Paint" program for PPC.
Of course I couldn't do without
Lexicon (the game)
Resco Word Game (like Scrabble)
TranCreative Wordbook (dictionary)
SpaceMaker (to recover space on my PPC)
Field's Software Print Pocket CE (for printing)
Strickland's Pocket Candy (screen savers for PPC)
try the trial versions to decide what you really will use
Alan