That is a tough question. Because i love the OS design, after all it was the more realistic of all the 3 styles (4 with the Reboot) and the one who started all.
My favorite part of the OS design isnt only the construction of the human characters, in a strange mix between regular cartoon animation and anime style of drawing/structure.
But also how they created every single one of the aliens in a way that AF/UA/Omniverse never find how to do. Vilgax, Six Six, Tetrax, Eye Guy, are only some examples of how realistic, good drawn and full of detail was the OS design. It was dark, mature and really simple in the same time.
Also the OS is the only one who used complex shadows and lights in the drawing of the characters and their animation.
So for me the OS was unique and really special. AF/UA try to copy/emulate the magic but they never find the way to do it right and only end up with a mess of design, because it become too simple, the OS never was simple, it look simple, but the detail in color, design and structure wasnt only really high but also really harmonious/integrated.
But i have to say i like a lot the Omniverse design, because it finds the way to make an AF/UA fan happy and give me at less what i always wanted to see in AF/UA but never found or saw anywhere, good designs. Ultimate Humungosaur is GREEN and BLUE FOR FUCK SAKE WHO IN THE HELL WAS IN CHARGE OF DO DAT CRAP...
And i wanted to see a good Ultimate Humungosaur design, something who look really realistic, dark and mature while in the same time goofy and simple, ironically AF/UA never give me that, give the best character development but really awfull designs.
That is why i have to say in my heart i have two favorite designs, my favorite the OS design and the one who put right all the wrongs of AF/UA and make me happy, while i finally could see the Chamcaster i love, the one who would never let Michael Morningstar be his boyfriend, the one who would hit his balls and send him to hell along Gwen, Hex and Adwaita like she did in Omniverse...