Hangouts was great. Respond to messages in browser, group chats that worked, and a well integrated video calling feature. It was essentially WhatsApp on steroids. Not to mention being able to reply with obnoxiously big emojis before that was a mainstream thing.
I got a whole bunch of friends to switch over to it only for Google to kill it off and replace it with...nothing? I use Messages+Duo now but they're far from a good replacement.
Google Now was another amazing thing that they killed off. That thing had a card which gave accurate predictions on journey time and notified you if there was unexpected traffic. The weather and news at a glance was really nice too. I changed launchers once it went away.