I personally like beans from Brazil (Santos or Cerrado). They usually have a smooth and mild chocolate caramel taste. There a many roasters offering these. Medium roast is fine for me, dark roast is more intense.
For Thai beans go for Chiang Rai beans. I have visited different plantations in the North of Thailand and consider Chiang Rai the best.
As mentioned coffee is a matter of personal taste, others might disagree with the above choices.