For 3rd party, PCalc Lite (EXCELLENT btw, added one .99 package otherwise free), Bring! (Like it for shopping so I don't have to walk around with my phone out while going down the shopping list--once left my phone in the cart and left the store...yikes that was scary; so glad someone was honest and turned it in!), and occasionally TuneIn Radio to control the station choice and play of my frequent radio stations while my iPhone is out on the countertop or desk for example and I'm doing things around the house. These are the only 3rd party apps I'm using on the watch now. I know PCalc is native, think Bring! is too. Off hand unsure of the other.
As for Apple apps I use the most: Heartbeat, battery, calendar, activity, weather are in my Glances. As for accessing as stored apps, I use Timer, Alarm (both are used as complications on certain watch faces too) and Messages..all the time. Same for Phone. And I do use the Friends Side Button all the time for contacting people. Love that it immediately gives me a phone or message choice for a person. And on phone choices I frequently have several listed numbers for each person and they are all just right there. Depending on the ambient noise at the time Siri may not always be the best choice for asking to make a call or to send a quick message. As long as my hand is free I find this a super fast way to access the contact. I get mail on my watch.