Although she's more than well represented in my collection, I'm not sure whether or not I actually like Nilsson's voice/role interpretations.  Strong and effective, yes, but I'd give a lot for a little more feminine vulnerability in some of her Wagner heroines. 

Hst, I have an interesting two disc set I picked up from MDC in the summer which shows her in some unfamiliar and interesting repertoire, taken from off-air broadcasts throughout her career.  These are probably the best recordings I've heard of Nilsson.

I don't think any recording really does her justice. I saw her only once, as Elektra at the Garden (cond. Solti, with Resnik as mum) and it was awesome.The five women at the start, right downstage, could hardly be heard above the din of the orchestra (brass overflowing into the stageside boxes). Madame makes her entrance way upstage, so far that there seems little chance she'll be heard either. But she faces front, opens her mouth, and that first "Alein!" cuts through the barage like a laser through butter. Not a soft feminine persona, I'll grant you, but a real titaness with one of those huge personalities that just fills the whole stage (Boris Christoff was another such). Voices for recording are one thing, but these big no-holds-barred stage voices are the real thrillers for me.