Ooo, that's a difficult question. I generally find infinites fun, or just being able to dish out a ton of damage with ares cannon. Out of non-infinites there's one that's technically speedrun viable but I haven't managed to make fast yet: Stacking Health Max Up then firing off a Hell's Heart with piercing. Some of my runs with it were actually pretty quick, but it's difficult to balance taking damage + not taking damage
