My favorite most used app is Apple Music. I also like that it has AirPlay controls built in, which I don't believe is accessible from anywhere else.
I use SimpleControl quite a bit as well. It is the remote app formerly known as Roomie Remote. It is excellent and can control complete media systems in seperate rooms of my house. I typically go to my iPhone for a lot of the controls, but during movies or just laying on the couch watching TV, it is easier just to flip my wrist and hit pause or play. It isn't always the fastest to load, but still quite convenient.
Directv also has a nice app, but I mainly use SimpleControl.
Overcast/Downcast - nice to control these on my watch, as well. Just wish I could get to AirPlay controls from the apps.
Wunderlist - nice for lists. You can find the list, hold down the screen, tell Siri to add an item and it shows up in the list. Very nice for my grocery list. It also lets you check stuff off as completed.
iDevices - Siri or direct control of lights (via a wifi power plug).
Do button - I have a WeMo and this makes it easy to turn it on and off.
Workflow has a couple useful features if you just want to pull up top stories from different sources. It has other features, but this one is my favorite.
Feedly is nice for a quick look at RSS feeds. Don't get me wrong, I don't read a ton of articles on my watch, I just like to look at it as an overview (kind of like browsing magazines at the store) to see if I want to pick up my phone/iPad.
TheScore - great for keeping up with my favorite teams. Sends me a notification 15 minutes before a game starts and sends another notification with the score ( a bit dangerous if you record games and watch later, though).
Fantastical2 - calendar
Carrot Weather
Twitter
Shazam
Ratings - Consumer Reports subscriber app. A to Z category list and it also accepts voice entry to search for product reviews. If you pull up a category like bacon, it will show their top rated brands. It typically has a pic of the product which can help you find it.
CNN is my news notification app of choice.
Arlo - security camera notifications of motion are also very nice to have on the wrist. Typically at work, I will ignore notifications on my phone, but having important notifications on my wrists means I will check them in a timely manner. I plan to also use Deliveries, but when I have motion at my door, it is typically a package. I still have to pull out the phone to check recorded video or a live feed, but it is nice to know when I should check.
Probably a few others I forgot, but there are many apps that I like to use. I don't just use the watch for notifications. I probably use it just as much as a remote control for Apple Music ( thanks to the nifty Siri feature and volume control) and podcast apps as anything...especially at work.