As everyone else has said, it depends on which language you think will be most useful or fun given what you would like to do in the future.
I would like to add that, if you are a westerner, you should know that Thai and Korean are “tonal” languages, i.e., the tone (like rising or falling in pitch, or both falling and rising, etc.) of a given syllable greatly affects its meaning. This is sometimes difficult for westerners.
Japanese is not tonal, but on the other hand, it has 3 writing systems, known as Kanji, Katakana and Hiragana, all rolled into one.
Whether you are intimidated or excited by the challenge is all up to you. I wish you well. Good for you for wanting to learn a new language. It will open up vast new vistas for you! Good luck!