I'd have to say John Collins was very much up there for me.
Slavan Bilic was a huge hype train. What a big disappointment he was.
In more recent times, Vlasic had such a hype after leaving us, don't think he carried the hype he had off us.
This thread got me thinking about Vinny Samways. What would the opposite of the hype train be? When the club spent over the odds for players the world could see were tosh.
(edit: We paid more for him than they paid for Klinsmann in the same...