Brad and Andrew, great comments on this topic. Thanks for getting involved!
I agree with Brad and Andrew regarding the naming of The Spinx, which I think was the greatest.
Second greatest, in my opinion, was The Vanishing Lady (De Kolta).
But "Pepper's Ghost" definitely would be very near the top.
I guess the topic of "pseudo-automata" covers a pretty wide range, but many of them (for example, those of John Nevil Maskelyne) must have been truly amazing, even though they may not strictly be "illusions."
In re-looking at my relevant post on one of my blogs, I see that there I was really addressing the two illusions which best represent Victorian-age conjuring. That may (or may not) be a significantly different issue! If anything, it may make the above answers stronger.
For instance, someone might say that "The Aerial Suspension" was a "great" illusion, but I tend to think that it doesn't well "represent" the Victorian era. But "The Sphinx" and "Pepper's Ghost" sure do!