Gooood question! I already asked that question here, and what I can say is:
What she really hates is spiders.
What she really hates are spiders.
The thing she hates most is spiders.
The thing she hates most are spiders.
All of the above versions are used, the problem is that I really don't know which versions are most used, common and sound best. I believe much depends on personal taste and regional usage. I myself tend to use a plural verb because it's what is ok in my native language.
Now let's wait for some native speakers.