Personally, I must say Navball Docking Alignment Indication (way better than the GUI version, at least for me), then MechJeb just to see how to rendevous from experience and not a video, and then Bluedog Design Bureau which adds parts. Lots of parts, around 250 currently. Then if you want a bit more, Stock Visual Enhancements and Stock Visual Terrarin, but since I don't know how powerful your computer is, only get those if you have 8GB of RAM.