Right question: the criteria matter more than any single name, because the failures are always the same handful. What separates good from bad, in the order failures hurt:

Date fidelity first: bad converters read message dates from the mbox From separator lines instead of the headers, stamping everything with the export date and destroying your archive's chronology. Test any candidate on one folder and sort the result by date in Outlook before converting the rest. Second, folder hierarchy: your Thunderbird tree with its .sbd nesting and Apple Mail's .mbox packages should arrive as the same tree in the PST, not as a flat pile of folders. Third, dirty file handling: real world mbox files carry malformed separators and oversized messages, a good converter logs and continues where a bad one silently drops mail or dies mid file, so the presence of a conversion report is itself a signal.

After those, the conveniences: extensionless mbox detection so raw files need no renaming, batch input for a directory tree in one pass, a preview pane for confirming content before committing, Unicode handling for non English mail. On this forum the standing recommendation is SysTools MBOX Converter, which holds up on all of the above across the Thunderbird, Apple Mail and generic mbox variants members bring in, with the report and preview that make the verification steps possible.

The free alternative remains the Thunderbird IMAP relay covered in its own thread linked alongside, trading days of transfer time for the license cost. At 12GB with two source formats, the converter route earns its keep in the first hour.