There are a lot of OSTs floating around, so how do people go about coding them to make sure the tags won't get corrupted during transfer process? Especially those with foreign language within the music title. I noticed some of my OSTS will retain their proper tags no matter how much I move them about, yet others that looked fine originally will turn into complete gibberish or a bunch of ???? if I so much as shuffle them between hard drives, OS, or have different system languages. What's the magic ingredient I'm missing?

While on that note, what's the best tagging software to use? Foobar tags don't always agree with iTunes or vice versa.

First of all you're confusing filename encoding and tag encoding, two different things.
For both Unicode (preferrably UTF-8 flavour) is the modern way to go.

For MP3s I use ID3v2.4 tags with UTF-8 encoding and explicitly disable the generation of ID3v1. v1 is deprecated and its functionality is too limited for modern use.

For everything else I use the native tagging format, like Vorbis comment for Ogg Vorbis and FLAC. Vorbis comment internally encodes all strings in UTF-8, so no problem here with any kind of language.

Concerning filename encoding and transfer between different operating systems. Well, you need a software that honors the encoding when compressing the files (and a archive format that can store the encoding) and of course your destination system has to do the same. ZIP is deprecated since it became a huge hack over the years. Both RAR and 7zip can store UTF-8 encodings (and probably a lot more) and therefore retain file encoding.

Here some Wikipedia articles, that could be interesting for you:
http://en.wikipedia.org/wiki/ID3
http://en.wikipedia.org/wiki/Vorbis_comment
http://en.wikipedia.org/wiki/Unicode
http://en.wikipedia.org/wiki/UTF-8
http://en.wikipedia.org/wiki/RAR
http://en.wikipedia.org/wiki/7z