This ABC logic is pretty annoying.
Wyyrlok > Zannah in Illusions because Wyyrlok brought Krayt to his knees. Lot of people seem to forget that at the start of the comic Wyyrlok states that his biggest advantage over A'sharad is that he knows Krayt. More so than any of the other one Sith, in fact if memory serves me right the Crosscurrent/Riptide novels state that Krayt has a telepathic connection with the First Wyyrlok(I admittedly don't know if this is passed on to the 2nd and third generation of Wyyrloks) But yeah, knowing A'sharad fears(Aurra killing his dad and things like that) Kinda helped, tremendously. Having such intimate knowledge about his Lord gave Wyyrlok a big, big edge against Krayt.
In contrast Zannah's illusion have brought out the biggest fears in her victims. Set Harth, Darovit and the rest. So, as an Illusionist I'd place Zannah above Wyyrlok just based on how much more developed and brutal her illusions are.
I assume that Wyyrlok held his own against Krayt for a decent amount of time, but i hardly believe that he forced Krayt on the defensive or anything like that. I'm not overly impressed with Wyyrlok as a duelist, so I'd pretty much give it to Zannah. Better mastery of her form and she has a good amount of knowledge on combat to know when to switch to a counter-offense or to a defense. Not to mention she's much faster and has some strength(not that much but i digress)
I'd give the victory to Zannah, 6-7/10 In my humble opinion. Her illusions are far more developed, her defense and counter-offensive impressive me more and her force powers are somewhat more lethal. But this is just my opinion.