Can someone recommend a few Joni Michell albums?

Just buy the 68-79 box set. You can thank me later.

She really has a great body of work. If we stick we the original question, it's got to be Blue,  then  Court & Spark as two starters.
The passion from so many here on a variety of LPs says much about her depth as an artist. There aren't too many artists who have passionate fans of what others might call her 10th or 11th best album! She is absolutely a benchmark in female vocals, folk, rock & jazz. Cheers,
Spencer

I would take the approach of getting one from the seventies , one from the eighties , 90's .... ect  . then decide which era you like and get more from that era first . Her album Dreamland is sonically incredible .