To claim you’re the best is a straight out lie 99% of the time. The development world is so immense, and there’s so much to be consumed, that no one singular person can hope to be the best at everything, or perhaps even one particular thing. Hubris is essential as you market yourself. Be open about your short-comings, the gaps in your knowledge, but then also inform potential clients how your mindset allows you to fill those gaps quickly to meet the needs of their project.
So it sounds cheesy, but just be your darn self. Skip the BS and get to what people will actually believe, and even more importantly, what is true.