In my opinion these apps should be installed in your everytime.
1.Whatsapp - This app has replaced year old messaging service, it is faster than any other messaging platform.The group feature, Audio call, Video Call everything is perfect in this app.
2.TrueCaller - Identifies those unknown numbers for you, If you are a girl you should never delete this app.This is very helpful in tracing who the person behind an unknown number
3.GoogleMaps - This app is for finding places, directions.If you are visiting a country first, this app should always be with you to find out where you have to go.
4.Googletranslator- It basically translates your langauge into any language, it comes in handy when you are in an alien country and you dont know their language.
5.Mouthshut.com app - For honest product reviews.
6.PDF reader - PDF reader is neccessary nowadays because this format is used for most of the documents you download on the internet.
7.MX Player - It is the best player to watch videos and movies.It is better than the VLC player.
Apart from these apps, you should ola, uber for booking a cab . Google Tez, Paytm to make payments from phone itself.