Open lead in a variant of Spades, where players must play a winner if able (CallBreak)
I made several AI agents that plays CallBreak. My current strongest rule-based agent uses the following lead:

If starting with 5+ spades, lead them: lead boss, otherwise lead lowest.
Lead sidesuit Boss.
Lead strongest below the second-Boss from sidesuit with a second boss.
Lead low from Short sidesuit.
Lead Trash - low from worst suit.


Terminology:

Lead - the first card in the trick.
Open lead - the lead in the first trick of the round.
Boss - Strongest card in the suit, based on the cards that were already played. For example, the King is the Boss if the Ace was played.