Well, for a scene to be the most ironic in Pulp Fiction, it usually involves a complete reversal of expectations. For example, like the death of Vincent Vega as I mentioned before. We expect him to be in control as a hitman, but he's so vulnerable in that bathroom scene. It's that sudden shift from what we assume will happen to the exact opposite that creates the irony.
In Pulp Fiction, a scene can be the most ironic when it shows a contradiction in the characters' values or actions. Take Jules' diner scene. He has spent most of his life as a violent criminal, but then he has this epiphany. His past actions are in complete contrast to this newfound moral stance. It's not something we would expect from a character like him, and that's what makes it ironic. It makes us question how a person can change so drastically.