It all depends on you as a user and what you're looking to do.
My PERSONAL top apps are...
Chrome - Browser
Lama - Profile controlling app (never again will my phone ring while in an important meeting)
Spotify - Music
Shazam - Music identifying app
Prey - Anti-Virus / Phone finding app
Where'sMyDroid - Phone finding app (Most people who steal phones look for Prey, and when they find it, they often assume that's the only program)
Chrome Remote - Remoting into my home computer. Great for when you're watching a movie and don't want to reach for the mouse/keyboard
Handscent - SMS app (GoSms was giving me too many problems on the S5)
Google Opinions - Free money for apps

Zedge - Ringtone finder
Again, this isn't the end all be all list, no list ever is. This is just my list of Must Have apps, that I tend to have installed on ALL my Android phones.
For more ideas, go to the Google Play store and look at the top apps. REMEMBER to look at the reviews for all devices as well as your specific device. Some apps are not worth the time of day or don't work too well. Also remember to take all reviews with a grain of salt.