If you using IC2 Power mostly, the IC2 Nuclear is the obvious choice.
If you using RF mostly, Big Reactors of some description is the most popular choice for late-game power, especially when you add in a turbine.
A passive-cooled big reactor is generally less efficient than the equivalently producing Active-cooled Reactor + Turbine.
It's usually not worth considering converting EU and RF between each other, at least in my opinion.
The most efficient choice would probably be a passively cooled Big Reactor. Turbines are even more efficient but more expensive and really unnecessary unless you need tons of power. Here is the online simulator for testing out designs, so you don't have to do it in world. There is a lot of configuration you can do with Big Reactors and what kind of design you go with depends largely on how much power you want and how much you value efficiency.
Alright, thanks.
Passive cooled means I just shove in the coolant and close the multi-block, and passive means I put water via piping, right?