[DrewBlank]

I would suggest picking the characters you like first and filling in your collection from there. The Secret Wars packs that are still out in stores have some of the more "fan favorite" characters (Klaw, Dr. Doom, Hawkeye, Wasp, Ultron,and the Wrecking Crew) and aren't likely to be recycled in later releases like the single pack figures.

If I were going to suggest rare figures, the ToysRus three pack with Firestar has been running a little expensive, and Jean Grey is the only non-variant figure in the entire line that I haven't seen at retail. A few Wal-Marts might still have the Gigantic Battles on clearance, but they've been getting rarer. X-Men Origins Wolverine are still on clearance a few places, villains are getting more articulation as the Spider-Man line progresses, and Iron Man 2 has some non-Iron Mans that are worth picking up, but you have to hunt through piles of movie armor figures for them.