Playing in Flight A-X of a sectional Open Pairs, both Vulnerable at MatchPoints in third seat you hold: (Flight A is 2000+ MPS; Flight X is 0 - 2000 MPS voluntarily playing up instead of at their own level - the up and comers.)
S:J9865 H:T9 D:53 C:AT94
and the auction proceeds:
P 1H P 2D P 2NT P 6NT all pass
The opponents are an unbalanced pair (possibly pro-client) with the stronger member on your right as declarer. (in other words, LHO maybe trained to get NT contracts into her partner's hand when possible/reasonable.) Their system is a fairly straightforward Two-Over-One card.
What is the best opening lead and why?
A blind lead from Jxxx is a well-known terrible lead, often killing your Jack. This is less true when holding a 5-card suit, but not irrelevant.
If partner doesn't hold at least the queen in one of the red suit opponents may be able to run 13 tricks once they are on lead - that would be a sure bottom if you haven't cashed the CA. Partner can hold a couple of Jacks and that still leaves opponents with a bare 33 HCP that runs 13 tricks once in.
RHO's 2NT rebid is likely showing any hand that might open a weak 1NT if playing that system; so in the range 12-14. It certainly is not showing any extra values overall, but may or may not be showing stoppers - sometimes it is best NOT to ask, because every question you ask also has the potential of guiding declarer's play.
RHO and I go back > 30 years, so there can be the possibility of a little personal rivalry in play - or not. He is a very good player with > 6,000 ACBL MPs, all earned and non bought, but LHO is likely weaker.