For clarity and fluency:
* Articulated: Used when someone speaks clearly and smoothly.
* Eloquently: Implies speaking with grace and skill.
* Fluently: Indicates speaking with ease and rhythm.
* Clearly: Means speaking in a way that is easy to understand.
* Precisely: Conveys speaking accurately and specifically.
For confidence and authority:
* Delivered: Suggests a confident and controlled presentation.
* Asserted: Implies speaking with confidence and conviction.
* Declaimed: Describes speaking with dramatic flair and intensity.
For casual and natural speech:
* Flowed: Conveys the idea of speech that moves smoothly and effortlessly.
* Chatted: Implies a relaxed and informal conversation.
* Spoke: A general term for uttering words, but without the negative connotations of "stumbled."
The best antonym will depend on the specific situation and the meaning you want to convey.