2♦ was multi, and you and your partner have agreed to bid aggressively. 3♠ was p/c, X of 4NT was undiscussed (lead a heart if they end up in NT?!). I'm no rocket scientist but I'm operating under the assumption that Righty has less than one heart or he snuck in a ♥A from another deck.
If I told you that there was only one suit to lead to beat the slam, which suit would you lead, and why? (If it's a coin flip, feel free to say so)

Help

2♦-P-3♠-P-
4♥-P-P-4♠-
P-4NT-X-6♥-
P-6♠-X-XX-
AP