I have no feeling about the market, but I think the 348 might be a candidate: Unlike the 308/328 (Magnum fame) and the 355 (best Ferrari ever at that moment) I think the 348 falls just in between the two. Looks are a bit like a Testarossa so that wont help it either. And the numbers made are large.
I also expect the F50 to fall in price. Same as above, it is situated between the F40 (Enzo's last) and the Enzo (the ultimate supercar).
Maybe the Daytona since there have been many around for sale the last year(s).
Probably the Mondial as well: it is getting more than 20 years old and will probably replace the GT4 as affordable first Ferrari.
And finally the 456 since the 456M looks very similar and the car is less popular than the 550 Maranello.
Just my thoughts. Let's get back on this one next year ;-)
Niels