I find that even with the massive libraries available for the iOS devices, there are only a few apps that I come back to every day more than once. Here's a quick look at some of them, and maybe they'll be different from what you use.
1. Reeder for iPad
Every day, I like to check my various RSS feeds on my iPad. It's usually the first thing I do before I get out of bed, and the last thing that I do before going to bed. For this I use Reeder as I have found it to be the best at managing all of these feeds. Reeder plugs into your Google Reader feeds and uses them to populate the app. What is great about Reeder is the way that it does this. The UI is very intuitive and takes full advantage of the iPad's screen real estate. The only downside is that you can only use it with your Google Reader account, so you can't just browse for new RSS feeds to follow and add straight from the app. Right now it's $4.99 in the app store, but it's definitely worth that, or even more.
2. Evernote for iPad and iPhone
Evernote has become my lifesaver for note taking in and out of class. If you don't know what Evernote is already, a quick summary of what the service is about is an online (or cloud) note taking system that syncs any notes you take to all of the devices you have its apps installed on, or just to the web service. The beauty of this for me as a student is that I can take notes in class on my iPad, but when i want to sit down and do some studying, they're already on my computer for easy access. As well with the iPhone app, I can take pictures, video, or voice memos, and also have them sync to the service. A great addition to the student, or working lifestyle.
3. Pageonce Personal Assistant for iPhone and iPad
I first discovered this app back when I was using a Blackberry (which feels like ages ago) and came to love the Pageonce service quickly. What Pageonce does is allows you to view many of your financial information, whether it be banking or bills, in one easy to find place. You just enter in your information (and don't worry, they are SUPER secure, when you first sign up there are about 3 pages of information about how secure they are, and being a bit paranoid about this stuff, I read through it all. You can also passcode the app itself to keep out prying eyes) and it will update the information whenever you request it. You can add your bank account, phone bills, paypal accounts, and almost any other account that involves transactions. What I really like about it is that it only allows you to view your information, you don't have any access to changing what is in your account from this app, so you don't have to worry about it falling into the wrong hands. If you're like me and like to know where your money is (and where it keeps disappearing to) this app is great. There is a free trial version, as well as an upgraded version with a couple nice features for $6.99.
4. Rowmote Pro for iPhone and iPad
This handy little app is essentially a remote control for your Mac which allows you to take control of any of the media players on your mac and control them with your iPhone or iPad. While this itself is great, there is a neat feature that I didn't even discover until about a week with the app that has made it the ultimate app on my iPad. With just the press of a button you can turn the Rowmote controls into an onscreen keyboard and track pad to control your Mac. So if you don't have a Wireless Keyboard and the new Magic Trackpad, you can just use the iPad as a both. You need to be able to connect over Wifi, so if your Wifi connection is really slow, you might encounter some problems, but for the most part this feature is great. As well as with Pageonce, there is a free version of this app to try first, and a paid version (which you need for the keyboard and trackpad) for $4.99
5. Air Video for iPhone and iPad
Like many people, I opted to save some money and go with the 16GB iPad with 3G. This is plenty big enough for most of my needs and desires, except for one. Movies. In comes Air Video to the Rescue. What Air Video does is sets up your Mac or PC as a video server, which will then broadcast the video folders that you assign, to your iPad or iPhone (though the iPad is much nicer due to the larger screen) over Wifi or 3G. The great part is that it will automatically convert the videos to the proper file format needed for the iPad without you having to worry about a thing. This way you can have all your videos on your iPad, without having to waste all of your precious space. As with most of the other apps, there is a free trial and a paid version for $2.99
So there you go! I hope you find some of these apps useful for yourself, and if you have a top 5 list, send it to me @attechstuff on twitter!