<p>For the first one, “data” are plural. The singular is “datum.” So it’s “tell” not “tells.”</p>
<p>The second one, I believe, is an issue of tenses. The words “thought” and “was” are simple past, so the “ever had” part should be in the past perfect, i.e. “had ever had.” (Weird, I know.) Alternatively, it could be “I think your chocolate cake is the best I have ever had,” but you can’t get to that without changing 3 parts of the sentence.</p>