The dealer has picked up the Ace of Hearts. What is your best lead?
The Queen of Diamonds!
First of all, the K and 10 should be quickly eliminated from consideration, because you should not lead trump into your opponent’s order. This is stated in the Sixth Commandment of Euchre.
Leading 9 in hopes that you could someday play the K and squeeze out a trick is a long shot. Your opponents could easily win a trick and lead another spade to allow the other player to Trump In. You will be force to follow suit, when you would rather play a trump.
Leading the K is an inferior version of the 9 lead. The K has slim chances of making it around the table, and the 9 is a poor squeeze card.
Leading the Q creates a void in diamonds. This maximizes your chances to Trump In if your opponents leads a club or another diamond. If you win a trick, you can still play the 9 and hope to squeeze the K.