My rarest LPs are probably a recording of Indian street music from Bengal, and a copy of a Gypsy Flamenco album recorded by Manitas de Plata (Little Hands of Silver). I found that one stashed in a cardboard box in the corner of a book store. When I saw it, I grabbed it and raced for the check-out desk!

I also value my 3 recordings by Steeleye Span, as several people here have already mentioned them.

For sheer sentimentality, my copy of Steppenwolf Double Live, the first LP I bought with my own money. My first 45 rpm (rememeber those) was Jan and Dean's Little Old Lady from Pasadena.