Hm... for me it's a tossup between Omicron Lost and Omicron Iota. Iota has some really cool nebula effects that arent seen anywhere else in sirius, while Lost has some unique lightning that I havent seen anywhere else yet.
Of course, there's also systems I've seen glimpses of that arent in the mod yet, and some of them are pretty damn cool =P
I love the Sigma's, hasn't changed with Sigma 19 still ranked at the top of my list. Our clouds create a great atmosphere and I love the ever-present lightning that reveals silhouettes in the distance. I love our green and blue stars. Over by Planet Hiran if you park on top of it the asteroid field in the southeast section makes a golden Halo.
Okinawa is also a place I am keen on. There is real nice spot I like on the eastern face of planet Miura if you turn left before going on the lane to Fujisawa, between the star, the other planet and the Nebula you can get a nice scenic view depending on your position.