Depends on what you do with your phone, here are the apps I feel worth paying for:
betterbatterystats - if your battery drains for no reason, this is a perfect place to start looking
mx player pro - I like the UI, better than free alternatives out there
pocket casts - the only app to listen to podcasts with pleasure
powerAmp - oldie but goodie, I do use 3rd party skin to mask it's ugly native UI though
relay pro - best reddit reader for me
sd maid pro - cleans Android like no other, very reliable
tasker/secure settings - must have for tweakers
solid explorer - no other Android file manager comes even close, make sure you get the material version
titanium backup - another must have for tweakers
tweetings - edged out fenix as best twitter client for me, fenix is still worth paying for though
weathertimeline - for weather apps, this one beats others to the punch
auto sync google drive/auto sync dropbox - sync google/drive or dropbox folders to your android
f-stop gallery - more feature rich than others I've tried
GMD Gestures - allows me to do anything on android with gestures
turboscan - if you ever use your phone camera to scan docs, receipts, this one is better than others