That depends on the user. Do you use star charts? APRS apps? Everyone has different "essential" apps, depending on their interests. Phone, Contacts and SMS (by whatever name) are almost always essential. I consider Hangouts Dialer essential too if you have a Google account with a Google Voice number. After that? I never use Google Play, Google Music, Flipboard (well, maybe once or twice a year, if I'm stuck somewhere and get tired of solitaire), so I don't consider them essential, although some people use a cellphone as an mp3 player that just happens to make phone calls, or a game playing box that happens to make phone calls, so they'd consider those essential. I consider the Walmart app essential, because I do my grocery shopping there, and I pay with my phone.
Diferent people, different essentials.