First off-- YAY, more Code Geass love! <3
In terms of which outfit, though, I'd say your choice should mostly depend on what you're comfortable making.
The emperor outfit is a really neat costume, but is obviously very complicated in terms of detailing. Most people I've seen do it also had to do some resin-casting, which can prove to be a little complicated. The school uniform is definitely a much simpler construction option, and is probably a good choice for someone with less sewing experience-- but like Stotch said, that's definitely a cosplay that's better with a group. I personally love
the Zero outfit, but there are particular elements about it that are difficult. The helmet could prove really tricky to do given the weirdness of the shape, and unfortunately, the official ones are pretty pricy
. If you can figure out the helmet (or are willing to go without it), I think Zero would work well for you!