BHIM app by NPCI
are two good UPI apps out there. Bhim scores for its low app size and simplicity (it is still buggy, but I hope the good folks who are making it with NPCI will fix them asap) while PhonePe scores for its elegance and good UX. Also, I guess PhonePe has released an iOS version as well.
Do check them out! If you are a merchant and have a bank account in India, you can instantly enable electronic payments. If you are an individual, you can send money from and to almost every bank account in India with these apps.
Strictly speaking neither of them is a bank app, and that shows the power of the UPI platform :)
Kudos to the NPCI team again though there is the question of conflict of interest in NPCI being both the pipe and the end consumer app. Ideally, the app should be spun off as a separate legal entity.