How did we not have a topic for this already?
What is it you liked so much about Republic Commando? Be it the game or the books.
Personally, I loved the level of control you got over your squad, which was totally unique at the time. It added a tactical aspect to the game that got me hooked.
I also liked the diverse personalities of the characters and how they all had very different roles dispite the fact that they were supposedly all the same. Both genetically and by upbringing.