My favorite time killing casual game is "Angry Birds".
As far as educational apps go? There's just so many.
"The Elements" (interactive periodical table) is a good app for demonstrating the unique interactive capabilities of the iPad (and other tablets), The astronomy apps, such as "StarWalk", the Music apps, such as "Magic Piano" and "Jam Pad", the medical apps, such as "3D organs"
I really like the "Appzilla" and Appzilla2" app They're .99 each, but contain about 100 different utility apps in 1, including police scanners, financial and other types of calculators, maps, instruments, all kinds of things.
Which ones are best though really depends on your own personal interests and what you'd like to learn.