Yes, I can see your point, and you have my sympathy! I wouldn't say any of these are outright wrong grammatically, just that some are more natural than others. Also, #5 and #6 are semantically different, because you are not expressing your like of English in them. A guideline is that each of "reason", "why" and "because" introduce an explanation for your liking English, but if you pile them up, then the construction becomes a bit redundant. Consequently, #7 sounds best to my ear, but it's also very common to hear "The reason why ....". One other point: one minor distinction in your examples is whether you use "that" or not. In this context, it's optional in English, and particularly likely to be omitted when speaking informally.

Thank you Kenny. This is by far the most detailed explanation I have ever seen. Thanks a lot for your input and hopefully we can schedule another time for practice this weekend. Have a good evening.

1. "The reason why I like English is that it is very useful“. - DON'T USE! sounds weird (too perfect), we try to not say "that" too much in order to avoid redundancy in spoken and written English. 2. "The reason which I like English for is that it is very useful. "- DON'T USE! Grammatically incorrect, (don't end a clause with a preposition: "for") also I never hear nor read this version ever. "which" doesn't work very well here. 3. "The reason for which I like English is that it is very useful." WRITTEN! Perfect grammar for written English, use in formal written situations, however in spoken English it sounds too rigid and unnatural, but excellent English! 4. "The reason why I like English is because it is very useful." PERFECT! Perfect English for spoken and written, it's not too correct that it is too formal or unnatural for spoken, but also not too casual or grammatically loose for even formal written settings. THIS IS THE BEST ALL-AROUND VERSION!! Great job. 5. "The reason is that English is very useful." DON'T USE! This doesn't explain your point, and the grammar is weird...: "that" 6. "The reason is because English is very useful." DON'T USE! Doesn't express your meaning, however is a good reply to something like..."What is the reason for the English language's importance?".. However still sounds rigid and unnatural, just forget it lol. 7. "The reason I like English is it is very useful." SPOKEN! Best version for spoken English. When we casually speak we like to use less words to express our point, unless you are trying to be poetic lol but perfect for spoken!