It's really hard to say, because each mod has a "best" option for whatever I am doing.
BC gives gates, which allow for you to create a highly customizable control system.
TE gives liquiducts, conduits, tesseracts, etc. which are so much more convenient/versatile than their BC cousins.
IC2 EU is extremely intuitive, nano/quantum is unbeatable, especially with full enchants. Also provides a viable endgame
TiCo doesn't really even need to be explained. Still farming them heart canisters.
AE is a HUGE game changer, saves me a lot of work and organization (OCD organization)
MFR is just so good for tech orientated/automation orientated players. Utilizing both EU & MJ, automated output, etc.
Forestry cause infinite energy, but still fun to build the system & bees.