Apparently Edward III was the first fluent English speaker but there's no reason to believe English was his first language, her Mother was French as the majority of the English nobility. One candidate could be Henry IV, he was born in England (not the first to do so though), but his Mother's family had been living in England for many generations, he was raised entirely in England and his first spouse was notably english, his second wive was Basque, maybe the first plantagenet King to not marry a French noble, he also joined the Order of the Garter, which probably was comprised mostly of English speakers.