Best word for nice? Friendly?
Which is better to describe someone as "nice"?
Amable? Simpático / simpática?
Then how about "friendly"?
Or are they interchangeable?
I believe they are all interchangeable. You can also use amistoso.
Simpático / simpática can mean nice, likable or even sympathetic.
I think I personally would go with amable or amistoso because I have heard them used in this context for friendly. But a native might have better advice.
They are not interchangeable. Amable is friendly, Simpático is cool (cool is also nice, but not in the same way as amable which is like, helping you out nice VS just generally being nice and cool like your friend might be) - he's pretty cool. Es bastante simpático. Your friend is simpático, the guy that helped you jump your car was amable and your friend's sister is agradable.