What is the best opening lead on this hand, and why?
I suspect you won't get a unanimous answer if you asked a number of top experts.  What follows is my guess.

If 6N was bid confidently, that should mean most of the field (since it's a fairly good field) is in the same slam.  I lead the Ace of clubs and take my 45%.

If 6N was not bid confidently, I lead a spade.  My partnership agreements are usually that non-honor leads at slams are meaningless (b/c declarer can usually use the info better than partner), so I lead a random non-honor.

If I am in a swinging situation where I am fairly confident I want top or bottom, I lead a diamond.

Updated to add explanation:

If opponents have 12 tricks no matter what, it's quite likely that they have 13 tricks outside of clubs, or at least they have 12 tricks and can squeeze me in the black suits for the 13th.  (This includes those situations where declarer has no choice but to finesse my partner's heart honor to get to 12 tricks as well as situations squeezing partner (unsuccessfully) in a red suit+clubs and me (successfully) in the black suits for the 13th.)  So I have to lead AC if I think they'll make.

If there is a way to beat the contract, it's quite possible leading AC sets up their 12th trick (for example if their clubs are Kx opposite QJxx).  Indeed, it's quite possible they have 10 or 11 tricks outside of clubs and can set up 1 or 2 more tricks in clubs.  Hence you need to hit partner's king immediately.  The heart king would be useless since hearts are probably with declarer.  That leaves spades and diamonds, with spades more likely because of the bidding.  Spades is also better because there are holdings where partner has the diamond king and the contract goes down except on the lead of the AC, whereas a diamond lead when partner has the spade king and nothing else probably means opponents end up with 1 spade, 5 in each red suit, and a club.

(But there definitely are holdings that require a diamond lead - most of these involve marked finesses against your Jack of spades.  Hence it's a reasonable swinging lead.)

(If one has to lead the 9 or 6 of spades, I think the 9 is better.  If opps have 7 spades (say A52 in dummy vs QT74 in hand), the 6 is more likely to induce declarer to try to drop the KS singleton (either in desperation for trick 12 or hope for trick 13).  Not that I'd think through this at the table.)

A low club hoping to hit partner's king or queen might seem like a good idea, but I very much doubt that LHO bid 6N without neither the ace nor king of clubs (even if 2N promised stoppers).  If partner has the queen and opponents need exactly one club trick, they are more likely than not to get the finesse wrong if you don't lead clubs but right if you do, since a lead of a club strongly suggests you don't have the queen.