My favorite Alexa skill is the ability to play music from my Amazon Music library. I can ask Alexa to play a specific song, album, artist, or genre, and she'll start playing it right away. I can also control playback with voice commands like "pause" or "resume." Another great feature is the Amazon Music Unlimited skill, which gives me access to a huge catalog of songs and ad-free streaming.
I also love the integration with my smart home devices. I can control my lights, thermostat, and other devices with just my voice. And, with the Routines feature, I can automate a series of tasks with a single command. For example, I can say "Alexa, good morning" to turn on the lights, start the coffee maker, and begin playing my favorite morning news station.
Those are just a few of my favorite skills--I'm sure there are many more that I haven't even discovered yet!