Music From Big Pink is a great choice. From the rock side of things I'd also have to consider:
Van Morrison "Moondance" - Can you imagine being there when "Into The Mystic" was laid down?
Led Zeppelin "Physical Graffiti" - You are at Headley Grange in about 1974 hearing a band at its absolute peak record their best album. If I could just be there at the recording of one song with Zeppelin it would be "When the Levee Breaks" and of course it's a tough choice to pick between Zep 4 and Graffiti.
Little Feat "Dixie Chicken" - Lowell George was one of the greatest voices in rock then throw in his guitar playing and wow. Would have been very cool to see him work.
The Beatles "White Album" - This is my favorite Beatles record and I think you gotta throw in a Beatles record if you're making a list like this but the thing is I saw the film Let it Be and so I feel like I've seen a Beatles album being made. It didn't look like a lot of fun either.
The Eagles "Hotel California" - I've met Don Henley and Joe Walsh. They were both great guys in person but the funny thing is when I see Henley being interviewed he comes across as someone I'd have no interest in ever meeting...I guess it was a good thing I met him before I ever saw an interview with him. I probably would have avoided him. When I lived out in California back in the 80's for a short time everybody I met that knew Don Henley did not like him. In fact to be honest they seemed to despise him. I guess I bring all this up because sometimes it may be best not to have been there sometimes and I wonder if I really would want to have seen 8 months of these guys. Anyway my brother and I played this record so many times between when it came out and the early 80's that we had to keep buying new copies of it because we wore the old ones out. As a side note at Thanksgiving my brother played me a bunch of songs that I guess will make up the new Eagles record. They were dreadful.
Captain Beefheart "Clear Spot" - Maybe not thought of as his best record by a lot of folks but it's the one I would have wanted to be there to see and hear being recorded.
Really this list would shift and change depending on my mood but if I were to just pick one recording it would clearly have to be:
Miles Davis "Kind of Blue" - Just read the list of musicians on this record and there really is no need to say anything else. What an interesting moment in time this must have been.
