1. Evernote - has many more options for notes you create, allows you to sync the notes so they are always up to date on all your devices. Allows you to edit notes on your computer which is way easier for typing and copy paste sometimes.
2. Twitter for iPhone, Osfoora HD(iPad) - These are the two twitter clients I use on my devices. I love keeping up with my timeline when I'm mobile.
3. Angry Birds - This game is so addicting, I now have it for iPhone, iPad and on my MBP.
4. Cydia(Jailbreak) - It's a qausi app it's the JB app store but it's a gateway to the apps. It is awesome because it allows so much customization of your device with themes, wallpapers, sounds and more. You can get many killer jailbreak apps like biteSMS, LockInfo, Activator, SBSettings. I have tried a theme that I am in love with on my iPad called iPad Blue I think by JMG.
5. Safari - This one is kind of a boring choice and it's not an app you have a choice for but browsing is so sweet and I do it constantly on my devices. There are tweaks and upgrades that could be made but it is a great browsing experience.