Best answer
It depends on what situation, really. In usual situation, to show respect you can say: “Sensei, doumo arigatou gozaimasu/gozaimashita.” And don”t forget to bow after that! Action speaks louder than words.
If you are leaving for good, or your Sensei is and presumably you will not see each other easily for a very long time, you can add “Taihen osewa ni narimashita” after saying “doumo arigatou gozaimashita. And again don”t forget to bow after that.
Answer
＿＿先生ありがとうございました。
＿＿sensei arigatou gozai mashita.
Thank you ＿＿sensei.
＿＿先生ありがとうございます。
＿＿sensei arigatou gozai masu.
Thank you ＿＿sensei.
＿＿先生、どうもありがとうございました。
＿＿sensei, doumo arigatou gozai mashita.
Thank you very much, ＿＿sensei.
＿＿先生感謝しています。
＿＿sensei kansha shiteimasu.
I am thankful for you, ＿＿sensei.
＿＿先生のおかげで、＿＿ができました。
＿＿sensei no okaged, ＿＿ga dekimashita
- because ＿＿sensei taught me / helped me, I could do ＿＿ very well.
- because＿＿sensei taught me / helped me, I exceeded in doing ＿＿.
＿＿先生にありがたいです。
sensei ni arigataidesu
I am thankful for ＿＿sensei.