You are assuming that messages have a uniform structure, with one well-defined “main part”. That is not the case; there can be messages with a single part which is not a text part (just an “attachment” of a binary file, and nothing else) or it can be a multipart with multiple textual parts (or, again, none at all) and even if there is only one, it need not be the first part. Furthermore, there are nested multiparts (one or more parts is another MIME message, recursively).
In so many words, you must inspect the MIME structure, then decide which part(s) are relevant for your application. If you only receive messages from a fairly static, small set of clients, you may be able to cut some corners (at least until the next upgrade of Microsoft Plague hits) but in general, there simply isn’t a hierarchy of any kind, just a collection of (not necessarily always directly related) equally important parts.