When the auction goes 1NT-3NT by the opponents, there isn't much imagination required.
Choose your longest and strongest suit and usually lead the 4th-highest card in it.
A few "what-ifs"
1) What if it is just about a "tie" for longest and strongest?
With two 4-card suits or two 5-card suits, roughly of equivalent strength, prefer a major when in doubt. (Your opponents would usually look for or show a major if they had one).
2) What if you have a "sequence?"
If 3 in a row (QJ10 or KQJ), lead top. If "almost" 3 in a row (such a KQ10 or QJ9), also lead top. Otherwise, tend to lead 4th best.