US English and UK English of accent which one that we should learn and better?

People will understand you either way, as long as you speak clearly. If you feel that you're more likely to go to the US or UK, choose that way. If you're staying in Cambodia, then you get to pick your preference. You also don't have to mimic native speakers completely. You can still pronounce the Ts in water, mountain, computer, etc. Anyway the English have some unusual pronunciation in some words too. Rs fall off where they are written and pop up where they are not written. I consider these things optional for ESL students.