Find My Phone is a must. If you ever walk away from your phone, your watch will vibrate to notify you. If you have your phone but walk away from your watch, your phone will notify you that you left your watch.
I've had many times where i left my phone in my car mount and i rush out of the car without grabbing my phone. I get about 30 feet and my watch vibrates reminding me. Invaluable.
+1 for Stay Lit Wear. One of those apps where you think "Why is this not a standard feature?" Since using it I've found little reduction in battery life. I always get two days from my Huawei Watch. Could probably push to 3 days if I changed my brightness settings and usage habits, but... naaah; 2 will do.
A few that I use:
Wear mini launcher
Stay lit, as mentioned above
Coffee, for SMS
Calculator (there are a lot to choose from)
Calendar for wear
Showear, to lock your watch
"Lock my phone", to lock the phone from the watch
Plus some "regular" apps that are useful on wear:
wearable widgets (I use it to have a widget that shows me my most recent emails), audio recorder ( for a quick note), feel the wear (to make vibrations longer), holo compass (if you have need for a compass), water drink reminder, wear charging (shows status of charging and gives alert when done), and wear volume (control volume from watch)