The Shadows (well, their ships) were the major reason I got into B5 initially, but as a threat they're vaguely simplistic (EVIL!!!) compared to some of the dynamics of the Dominion...nevermind that those dynamics never got to the point of fundamentally altering the direction of the story.
I call it too close to call.
I have to disagree with this as well. The Shadows were far from one-dimensional, or evil. As with the Vorlons, they have a philosophy that says their way is the right way. Other races will evolve through chaos and disorder. They are helping the universe by introducing the needed chaos. Without them, other races would stagnate and, if anything, de-evolve.
The Vorlons believe in order. They set the rules that others must follow, and are happy to manipulate other races, right down to their genetic structure, to make that happen. They have been manipulating races for millenia, creating racial images and memories that make beings see Vorlons how they want to be seen, inspiring awe and worship.
Neither side is evil, neither side is good. They simply are. Contrast that with the Dominion, which simply wanted to destroy everything that was not like itself. Now those folks were one-dimensional. Ironic for shape shifters.