I agree that Castro took the most advantage of the opportunity of this particular debate configuration. The candidates I would say who missed that opportunity the most are O'Rourke, who didn't get much speaking time, and Klobuchar, who keeps acting like she's running for president in 2000 (or at least tome). Klobuchar's talking points about immigration and her closing statement about how she's differentiated from the other candidates weren't especially bad or inappropriate – but they didn't seem directed at the larger conversations that the other candidates were having.