What would be the worst thing in the world for Winston?

When Winston and O'Brien are talking about Room 101 in
Book 3, Chapter 5, O'Brien tells Winston that the worst thing in the world is in that
room.  He then goes on to tell Winston what that means in Winston's specific case.  As
he says to Winston:

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'In your case, said O'Brien, 'the worst thing in
the world happens to be
rats.'

We have seen before
that Winston has this horrible fear of rats.  Having seen this, we are not surprised
that this is what is in Room 101.  But it is still pretty horrifying because of what
O'Brien says will happen to Winston.  Basically, they're going to put a rat in a cage,
strap the cage to Winston's face, and let the rat
go...

'You
understand the construction of this cage. The mask will fit over your head, leaving no
exit. When I press this other lever, the door of the cage will slide up. These starving
brutes will shoot out of it like bullets. Have you ever seen a rat leap through the air?
They will leap on to your face and bore straight into it. Sometimes they attack the eyes
first. Sometimes they burrow through the cheeks and devour the
tongue.'