Depends on what you're looking at. If you're talking just about general attractiveness, Halo 4 Cortana without a doubt. In terms of story, however, Cortana seemed like more of a mother-figure/guardian in Halo 3 whereas in Halo 4 she seemed like a close friend almost to the point of being romantically involved. Both of their respective designs fit those different roles perfectly in my opinion. I think what really matters is which role that's preferred. I think I like the motherly Cortana a little bit more, and I also think you're weird if you think the motherly Cortana is attractive, because her motherly role is matched perfectly with a motherly appearance. Also, pre-Halo 3 Cortana is nothing special. Nostalgia goggles off.