Can someone recommend a few Joni Michell albums?

Blue is indeed widely regarded as her classic album.
It's available as a HDCD disc on the most recent remaster.
It's pretty much straight ahead Joni songwriter style with acoustic guitar and piano prominent.
It's reflective in mood and was apparently influential on Dylan prior to Blood On The Tracks............

"Miles of Aisles" is a double LP that has many of the most loved Joni songs, recorded in a live performance. The recording is a good one, and the crowd doesn't really interfere with the songs. The song "All I Want" on this LP is IMO the best version of that song she ever did. If I could keep only 1 Joni album, this would be it. There is one song that did not appear on any Joni LP, only on 45rpm single, called "Urge for Going", that is an absolute must-have song for Joni lovers. It is my favorite, out of all of her songs. A CD recently came out called "Joni Mitchell - Hits"(Reprise 9 46326-2) that has "Urge for Going" on it. It is the 1st song on the CD. I have that CD, and I also have an original pressing 45rpm vinyl single. Do not miss this song! It is her most beautiful work.

I second "Court and Spark". It's one of my all-time favorites. "Hissing of Summer Lawns" sounds about the same, but not quite as good.

Regards
Jim

My favourite is not one typically found on the must have list. It is "Mingus", yes, a tribute to the man. Very Jazzy.

All of the above recommendations are good ones and do not miss the little known 'Ladies of the Canyon', it is very mellow and a little jazzy, it is one of my favorites (even though I agree all the others mentioned are great too).