I've never seen a Mac Plus, or 512 "ED" version, but from the pictures I've seen on Google, there is no technical differentiation between the "ED," and a standard 512/Plus, with the exception of the badge alteration. I have heard of a 128 ED, but I'm not sure if that would constitute as rare, either.
Rarity, in regards to the compacts, would be identified more with add-on features. For instance, one of my Pluses (which was either a 128, or 512- I can't tell because the original serial number was removed), has a BNC adapter at the back. I thought this was rare, but according to Mac128, these BNC adapters were pretty much commonplace with the early compacts. Unique, nevertheless.
The Radius video card for the original SE is seen as rare (and, of course, the transparent case SE). I would need confirmation, however, from the 68k forum to see if this is in fact true.
Other than that, I would say that the 512ke is probably the "rarest" of the compact series, as it had the shortest product lifespan, in addition to the original Macintosh (before it was re-badged as the 128k).
Rarity, in regards to the compacts, would be identified more with add-on features. For instance, one of my Pluses (which was either a 128, or 512- I can't tell because the original serial number was removed), has a BNC adapter at the back. I thought this was rare, but according to Mac128, these BNC adapters were pretty much commonplace with the early compacts. Unique, nevertheless.
The Radius video card for the original SE is seen as rare (and, of course, the transparent case SE). I would need confirmation, however, from the 68k forum to see if this is in fact true.
Other than that, I would say that the 512ke is probably the "rarest" of the compact series, as it had the shortest product lifespan, in addition to the original Macintosh (before it was re-badged as the 128k).