I usually prefer my burgers to be cooked at Medium. Oops, sorry. Got a little mixed up there for a sec. My bad - must be a little hungry or something...
Anyways, welcome to the world of POTJ
James! It's a beautiful line of figures. I'm sure you'll enjoy them!
As for as the toughies go:
The hardest by far is the Masters of the Dark Side (MOTDS) Vader/Maul 2-pack. It'll run you at least $30 most everywhere.
As far as basic carded figures go, I'd say the 3 toughest ones to find were probably the clean Biker Scout, Sandtrooper and R2-Q5.
IG-88, dirty Biker Scout and FX-7 were somewhat hard to find for some people as well.
Not sure if you collect carded or loose, but if it's loose, you'll probably be seeing a few more of the HTF POTJ figures get repacked, as has already been down with Teebo, and Eeth Koth (though the RFT and IO are technically different as far as the head sculpt goes). So there very well could be some more POTJ rehashes showing up in the Saga line in the near future.
Obviously you should check out the various trading/classifieds boards, and maybe throw "POTJ lot" into your eBay favorite searches and hope to score a nice lot of them all at once. Most of them can still be had for regular retail or less, but some of the more popular ones, as well as the ones I listed above might be a few bucks more.
Good luck with your search James and keep us posted on any great deals if you land 'em!