Of course, there are way too many to fully rank, but at the very least, do you guys have a top 5. Interesting thought i just had and it's pretty difficult but here's mine.
Chuck Rainey - This was really hard to decide but, here it is. Played almost the entire Aja album with the exception of 'Deacon Blues', and had an incredible groove on most of the Royal Scam tracks as well as the two tracks he did for Gaucho and Katy Lied. What puts him over the no. 2 slot is how each of his bass lines (at least for Aja) are not only pivotal, but memorable. Black Cow, Peg, Josie, and Home at Last all are examples of this.
Larry Carlton - Going between him and Chuck were incredibly difficult. Carlton of course could potentially be put on top just for his solos on 'Kid Charlemagne' and 'Aja' alone.
Jeff Porcaro - Pretty telling that Jeff is apparently the one on most of the demo recordings, on top of all the actual studio cuts he ended up on. When Walt and Don had an idea, they had Jeff to set it to a rhythm.
Denny Dias - It took a while to argue whether to put him on the list given he was an actual member of the band before it became studio-exclusive, but just remembering that Dias was the only one of that original band Donald and Walter kept bringing back in the studio to contribute says a lot about his musicianship and trust the two had in him.
I had to stop at 4 because it became really difficult to try and argue between the tens and tens of session musicians W&D used, but Hugh McCracken, Dean Parks, Bernard Purdie, and Steve Gadd all contend for that 5th spot. Purdie, if you make me choose one from those 4.