What is the best teaching method for learning English?
 
According to academic research, linguists have demonstrated that there is not one single best method for everyone in all contexts, and that no one teaching method is inherently superior to the others.
 
Also, it is not always possible – or appropriate – to apply the same methodology to all learners, who have different objectives, environments and learning needs.
 
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Method   Focus     Characteristics
Grammar Translation  Written literary texts 
Translate from English into your native language
However, it does little to improve your ability to use the language for oral communication.
Direct Method (also called Natural Method) Everyday spoken language 
Student learns by associating meaning directly in English
 At this time teachers were starting to experiment with teaching and educational models as previous techniques were failing to improve spoken communication.
Audio-Lingual Method Sentence and sound patterns 
 Listening and speaking drills and pattern practice only in English
Cognitive Code Approach Grammar rules 
 English grammar rules deduced and then understood in context
The Silent Way Student interaction rather than teacher 
 Teacher is silent to allow student awareness of how English works
Suggestopedia Meaningful texts and vocabulary 
Relaxed atmosphere, with music; encourages subliminal learning of English
Community Language Learning Student interaction 
Understanding of English through active student interaction
Comprehension Approach (Natural Approach, the Learnables, and Total Physical Response) Listening comprehension 
 English speaking delayed until students are ready; meaning clarified through actions and visuals
Communicative Language Teaching Interaction, authentic communication and negotiating meaning 
 Understanding of English through active student interaction; role play, games, information gaps
Content-based, Task-based, and Participatory Approaches What is being communicated, not structure of English 
Content based on relevance to students’ lives: topics, tasks, problem-solving
Learning Strategy Training, Cooperative Learning, and Multiple Intelligences How to learn 
Teach learning strategies, cooperation; activities vary according to different intelligences

 
 One disadvantage of using ONLY one method is that other methods may have helpful approaches FOR YOU. Your skills and goals are certainly different from everyone else's. As an adult learner, a simple explanation (in your L1, or other language you speak well), a simple answer might save hours of seeing and hearing examples. While children acquiring their L1 won't benefit from grammatical / analytical explanations, you might.
My teaching is probably best called "eclectic" -- using different methods for different situations and students,

思えば英語ではGrammar Translation Direct Method  Audio-Lingual Method をやったな、学校や塾で、昔。
どの方法も一長一短あるもんね。
色々なことをやるのが良いのかな、やっぱり。
 
The modern teacher will in fact use a variety of methodologies and approaches, choosing techniques from each method that they consider effective and applying them according to the learning context and objectives.
 
They prepare their lessons to facilitate the understanding of the new language being taught and do not rely on one specific ‘best method’.
 
現代のteacherは大変ですね~。