Note: this is for the oldest version of the game (1.0, I think, since mine is from launch day).
Here's the master code I used (from Codejunkies):
[spoiler]
C60CC12C 800018A8
042663A0 000034E0
042663A4 000034E4
04005614 60000000
04005C24 60000000
04005D50 60000000
04036598 60000000
04036740 60000000
04005C24 60000000
04036688 60000000
[/spoiler]
I also made some codes, but they're not all that useful:
[spoiler]Displayed HP Value Modifier for 1st Party Pokemon:
043A1C38 XXXXYYYY
...2nd Party Pokemon:
043A1C68 XXXXYYYY
...3rd Party Pokemon:
043A1C98 XXXXYYYY
...4th Party Pokemon:
043A1CC8 XXXXYYYY
...5th Party Pokemon:
043A1CF8 XXXXYYYY
...6th Party Pokemon:
043A1D28 XXXXYYYY
Explanation: These codes don't modify the actual HP values. They modify the values that are shown for current and maximum HP in the party screen
How to use these codes: X is the displayed value for the maximum HP, and Y is the displayed value for the current HP.
[/spoiler]
Edit: Made some "grammar" corrections.
Edit 2: Added version of the game.
Edit 3: Added credit for the master code.
Edit 4: Fixed some problems with the codes.
Edit 5: (Yes, Edit 5) Made some minor changes.