Completely depends on what you're looking for. Games? Productivity? Health and fitness?
Do not forget travel. That so many travel apps are free does not mean that they are not worth paying for.
Most of the priced apps for travel are destination specific. I find that the $1-5 apps for a country or city are much better values than a guide book. Going to Denmark next month, already have the app. Like many of these apps, it is location aware. So, it really has two uses, one for planning and one for navigating and finding.
Perhaps my favorite travel app is iMap, a free app that meets your test of "worth paying for." While the app is free, one may a premium for connectivity in foreign destinations. So, in preparation for my trip to Copenhagen, I will sign up for an Xcom Global MiFi card, in part because it allowed me to use iMap in London.
I am reminded of my first cell-phone, analog, the size and weight of a brick, $1500 for the phone and a $1/minute for connect time, plus roaming and tolls. I hardly used it for any of the purposes for which I pretended to justify it. However, I discovered dozens of uses that I could not have anticipated. The same is true of my iPhone and my iPad. They are life changing in ways that I could not anticipate until I had them. I am still discovering life changing apps every week or so. The value blows away even the best personal computer and, of course, that first cell phone.
Go to YouTube, watch Bridger and think about what Steve hath wrought. Do not be fooled into believing that just any tablet will deliver the value of an iPad. The iPad is what it is because it followed the iPhone. They are both what they are because of the apps.
The apps are to the iPhone/iPad as Morse Code was to the telegraph and Steve Jobs will rank in history with Morse, Goodyear, Whitney, Ford, Bell, and Edison. He has enriched us all.