All ways are best, as long as they give you what you want.
An image based height map with a large texture stretched over the entire terrain is best… in some situations. A pre-modelled landscape mesh with a repeating texture is also best… but in other situations.
All methods has it’s pros and cons, and it’s up to you to decide what you want, and what you are willing to sacrifice to get it. Without telling us what you want (and don’t want), we can’t say what you need.