Nova Launcher, TeslaUnread, Google Messenger, Percent Enabler
I use CalcU as my main calculator, and I disabled the built in calculator.
I use a couple of apps as companions rather than replacements to the built in apps:
Kit Kat Calendar... So I can change the status of events as either 'Busy' or 'Available'. The Google app ignores this setting and defaults new events as Busy. Meetings in Moto Assist only quiets the phone if you mark the event as Busy. So if I record an event such as attending a baseball game, I can label that event as Available, since I don't need a silent phone.
Shuttle Music Player... to manage playlists of locally stored music. I mainly use Google Play Music. But after the last update, my playlists disappeared that contained locally stored music from the music folder, and it ignores those songs in new playlists that I create. Its weird, it can still play local music, and I can add them to a playlist, but when I re-open the playlist in Play Music, they're not listed.