These are the core add-ons for the stock game that add more realism, but don't mess with the tech tree or add a ton of parts. The core game plus it's expansions have a boatload of parts now, and I find most everything I need in it.
Certain addons are not currently available on Spacedock, but can be downloaded via the CKAN app:
MechJeb 2
Waypoint Manager
Strategia
PlanetShine
Kopernicus can be downloaded from it's GitHub page: