I should start by telling you you're using the wrong verb: you want 'to teach'
To Teach --> taught (past tense)
To Think --> thought (past tense)
It's not a mistake to feel bad about though. There are more irregular verbs in English than regular ones. Even in what I've said so far, there have been six irregular verbs.
Onto the question. You have three sentences. The first one is basic past tense:
You taught me English.
This means you were taught by the person in the past, but were not being taught anymore. This is what you want to say, and is correct to use.
The second sentence uses the pluperfect:
You had taught me English.
The pluperfect describes things that happens a long time ago in the past, but is normally only used when another event happened more recently in the past, and here you haven't added one, so it wouldn't be the best choice to say this. However, it is correct and natural.
Your third one uses the imperfect:
You used to teach me English.
You shouldn't use 'would' as you mentioned in the example, because that is the condition tense and is not correct to use here. Also, remember to say used (pronounced yoossed here) instead of 'use' for the imperfect tense. The imperfect tense describes something that happened regularly in the past, but not anymore, and would be correct to use.
To finish, you can say 'You taught me English' or 'You used to teach me English' in this case and both would be correct and natural.