I'm a huge fan of Star Wars and Halo. I have to say I like Star Wars more as I have been a fan longer. I also know a lot about Star Wars. It seems tough between these two. Halo have the Flood, Halo Array, Shield Worlds, and the OP Precursors. Star Wars have the Yuuzhan Vong, Zonama Sekot, extremely potent force users (Nihilus to name an example), and there's always wounds in the force and Abeloth (also OP). One thing is certain, the battle won't be decided by conventional warfare so throw all that out the window. I'm going to vote Halo for now because of the Precursors, but that is subject to change if I remember something in the Star Wars universe that I'm forgetting.