Depends on what kind of "rare" you are after. Pretty obviously you must mean real physical copies, but also consider:
1) Rare, as in, hard to find an original
2) Rare, as in, hard to find a first-edition (non-budget) original
3) Rare, as in, hard to find an original that seems to work & in good condition
4) Rare, as in, hard to find a *good* original (unmodified, and with no errors)
People have pointed out that what we consider a rare game to be "strange". Indeed, what we consider rare is not actually be the same "rare" as most people think of it. A case in point, very nearly all copies of The Secret of Monkey Island that we have come accross are modified (due to save games) - so to us it is a rare game, but most people would probably say this game is rather common.
The point is - it depends on why you want them.