Not really a rating – but like Exodus mentioned in this answer offering help to decide trustworthiness yourself:
Appbrain shows which libraries an app does include, divided in "Ad network libraries", "Social libraries" and "Development tools". Except for ad libraries this still means you need to know what a library does, but for many libraries the name makes this clear – unfortunately not for all. They are linked to detail pages, but those focus more on developers to know how "well known" and "wide spread" a library is than on end-users who are rather interested in how much it invades privacy.
If you're looking for apps to cover specific needs you have (such as the OP who just switched to Android), you might find my Android site helpful: IzzyOnDroid's app listings have apps sorted by categories, sub-categories and, at the lowest level, groups – where relevant apps are then "grouped together". Next to each app you see how many permissions it requests, and how many invasive trackers it contains – linked to specific explanations. So you can already determine in the "overview" which apps you'd rather avoid (e.g. due to too many trackers). Information on trackers by the way is collected from both, Exodus and Appbrain. Plus sometimes even other sources.