Of all musicians, alive or dead, you get to choose as many as you want for one big hour long jam in your parents’ garage. You have to play with every musician for the entire hour, so no 15 minutes with The Who and 30 minutes with the Berlin Philharmonic and 15 minutes with Mozart.
Oh, and what is your instrument?
I play guitar, and I would spend that hour with Ginger Baker and Jack Bruce.
2nd choice would be with Miles’ band from the early 80s, but I would feel woefully inadequate.
Since I have no talent whatsoever, I’d have to choose Milli Vanilli. I think they’d understand.
(Guitarist)
Hamburg era Beatles.
Alive or dead?
Alive, either the Steve Morse Band or the Dixie Dregs.
Dead, Randy Rhoads and the Blizzard of Ozz.
Slee, guitarist
Of course I’d be way out classed but it’d be fun.
I think if I was going to jam with a guitar god like a Randy Rhoads or a Steve Morse or a Jimi Hendrix, I’d have more fun switching to bass and just keeping a groove while they do their thing. I think I’d rather try to do something to enhance their playing than to try to compete with it (which would be impossible).
Deleted my original as with mature reflection, it would be Muddy Waters. Just so I could play the rhythm guitar on Mannish Boy.
I used to get to play fairly often with one of the NW ‘guitar Whizzes’ who could just burn down the house on Hendrix tunes (you folks from the Portland, OR vicinity can probably guess who I mean). I can fake a pretty fair Noel Redding, so an hour with Jimi & Mitch is all I ask. Why can’t we have those guys back? Big Sigh
I’ll play harmonica with you. Muddy Waters was my choice as well.
Jimi Hendrix; Joey DeFrancesco; Dr. Lonnie Smith; Dr. John; James Booker; Don Patterson; Jim Hall.
None of these guys would have any time for me or with me, but I’d learn a bunch just trying to hang.
For the NW guy – I’ll add Leroy Vinnegar and Janice Scroggins and Thara Memory – and Nancy King on vox. Probably hardly nobody who doesn’t live up here knows these people well, but they all are (and, in one case were) at the very height of musicianship. I play just keys – Hammond and pianos, as per the initial query.
Einstürzende Neubauten and I’d play blocks, not Chinese wood blocks but those big cement blocks with a masonry saw.
Or alternately Christina Aguilera where we’d drill the U.S. National anthem. I’d play a studded leather paddle.
I play keyboards mostly. I’ll take Bonham and John Paul Jones as my rhythm section. I’ll throw in Hendrix and Stevie Ray Vaughn to trade rhythm and lead guitar roles. I’d try to throw Jimmy Smith in there on the B3, and just lay back, relax, and watch the show. If I can’t do that, I guess I’ll hit the piano, although I’m not sure how I’d fit it in all of that rock noise. Can I just be a Bez and dance around with a tambourine or something?
John Fogerty. I want to play Fortunate Son and Proud Mary with him.
Vocalist - I can range from medium alto to high end soprano. I can also “play” Flake’s keyboard presets while he goes out and gets abused by Til Lindemann 
Rammstein has a couple songs that have great female parts [Engel and Stirb Nicht Vor Mir] and they have a few other songs that could be turned into duets.
Mozart. No question at all. I want to pass Wolfgang my XV-585 and stand the hell back.
I’ll keep my strat and see if I could keep up after ten minutes.
Carla Bruni.
Accompanied on organ.
Holding a guitar with Ritchie would be contemptible so I would sit with Blackmore and have a beer.
I play guitar but suck at it so I pick EVH just to watch him.
Flute with Ian Anderson
I second the Jethro Tull guys, They seem like they could integrate a jam of any instrument.
Also, Django Reinhardt would be awesome to join.
Lastly, I don’t know if Summer Glau plays anything, but she could just repeat A - E - C over and over again and it would make a memorable jam session.
I saw Blackmore’s Night a few years ago at 4th and B in San Diego. Richie was handing out beers to people in the front rows, much to everyone’s delight. Not only that, but he was smiling throughout most of the performance. I wondered at one point, “Is that really Ritchie Blackmore?”