I have no clue what was rarest but silken floss was a figure I had 1 chance to order at retail and it was gone.
The aforementiond brando jor el I would agree with being hard to get. At that time in an not even sure sideshow distributed most hot toys figures. I had to get a couple through toys2.net. The jor el I missed out on.
Gun metal iron man has also got to be near the top. I probably am confusing it with another figure but back in the day, when sideshow put those up for order and you had to log in to a priority pre-order page, I seem to recall the gun metal gray iron man being sold out in an hour or so.
Now most hot toys are pretty easy to get a hold of.