It is perhaps helpful to think of each type of lead as having a base utility (out of 10). Ace from ace-king would be 10, a singleton 9, king from king-queen 7, away from a king 2, away from an ace 0 etc. The bidding alters that utility number.
For example if the opponents bid your singleton suit, its utility goes down from 9 to a 7. It may still be your best lead, but it may not; if you have, say, a queen from queen-jack, a 6 to start with, that 6 would increase to 8 if it was the only unbid suit, so overtaking the singleton.
For starters, let’s assign an initial utility to the following holdings (assume it’s a trump contract, and these holdings are not in trumps):
If the auction is entirely unilluminating (as this deal’s), you would lead according to the above utilities.
|1 ♥||Pass||4 ♥|
Misassessing the initial utility of the ace from ace-king lead, relative to the singleton, West led ♦ 3. Declarer won ♦ J, drew trumps,and cashed ♦ AKQ. 10 tricks and game made.
What should have happened
Ace from ace-king gives you a free look at dummy and thus keeps all your options open - you can always switch to diamonds. On ♠ A lead, East gives a discouraging signal of ♠ 2 (throw low means no). Given dummy’s strength in the suit, West does not switch to diamonds, rather to clubs. East scores ♣ AQ and leads a spade to West’s ♠ K. Down one.
If you remember one thing...
The Initial Utilities of various holdings for the opening lead.