Some utility mods that don't change the game:
My No.1 mod: Kerbal Engineer Redux or short "KER" - it does all the important performance calculations (Thrust-Weight-Ratio, Delta-V/Range) you need for the more complex flights.
Kerbal Alarming Clock and Transfer Window Planner are no must have mods as the former one, but planning and execution of your interplanetar flights becomes much easier.
NavBall Docking Alignment Indicator is a small and handy mod which helps a lot when... well .. docking :-D
Post edited March 16, 2018 by MikeWerner