"I like that one."
Translation:Dw i'n hoffi hwnna.
That is actually almost OK, ie Hwnna dw i'n hoffi (that one I like) is almost correct and would be heard in oral Welsh as an emphatic sentence. I've added it as an alternative answer.
However to be 100% correct it's necessary to add an extra pronoun Hwnna dw i'n ei hoffi ((it's) that one I like it)