Decks I actively want Ballista in:

-B/G Constrictor. Little surprise here.
-B/G Delirium. Two card types for the graveyard, flexible drop is appreciated as are the ping(s) it provides.
-Pretty much any single artifact-themed list in Grixis colours; preferably more aggressively-slanted ones. It doubles down as synergy enabler, can ping a chumpblocker or two and doubles down as reach.
-Finally, Mardu Vehicles; though I admit Ballista serves as more of a roleplayer here and could be cut.