Hipmunk, Google Flights, Airbnb and Booking.com for all the basics. Second Duolingo for learning basic phrases.
I’ve found Booking.com more user-friendly than Tripadvisor, which seems to overzealously apply filters for me, and has too many activity ads nowadays.
Despite the plethora of apps out there, I still love using Lonely Planet guides. I’m mostly wifi-free when out traveling for leisure, so they are the best option for those times when I’m out and about.