The best apps really depend on what you are interested in. I'm a music nut, so for me Shazam is a must. If I hear a long on the radio or in a store, I can find out what the song is called and who the artist is by having Shazam 'listen' to it. Shazam can also direct me right to the artist's itunes page.
Another great app is Jango. It's like a radio app but so much more. You input artists or bands that you like, and Jango will play them and other bands that sound similar. As the listener you can give each song a thumbs up or down. Jango learns what you like and custom-creates your station based on your likes. It's a great way to find new music!
I love whatsapp too! I also like to use LINE where I can make free calls using wifi connection. It's something like Skype but more fun with its chat interface...you can download stickers with various emotions and expressions (sometimes they speak louder than words)