Which font extension is better to work with XeLaTeX
First let's have a look on all the files. Here is, what I guess they are (but I am no expert here...). Please also have a look on Mrs. Beeton's comment below which discusses some of my misinterpretations and gives more information on certain extensions:
- afm: Adobe font metrics file
- enc: Encoding file
- map: mapping which tells TeX how
.tfmfonts relate to actual type1/truetype/metafont fonts - pfm: Printer outline metric font file
- tfm: TeX Font Metric file
- ttf: TrueType Font file
- pfb: Type 1 PostScript font file (binary)
- vf: TeX virtual font file
The question on .ttf vs. T1 is treated here and for sure somewhere on this site as well.
.vf files are roughly TeX font metric files for 'virtual' fonts. These may be composed from characters taken from different fonts. For example, you could create a font including oldstyle figures using letters from a regular font and figures from a supplementary font. They may also tell TeX how to construct characters not present in the original font, including ligatures (e.g. ffi) and accented characters (e.g. é). Like .tfm files, they also include metric information needed for bounding boxes, italic correction, the use of ligatures (e.g. f + i -> fi) etc. Please see here for very good information on this topic.
Finally as a clear answer to your question: You should use the .ttf file here. As XeLaTeX is able to handle these fonts, you should use it. Even better would be .otf, but I do not want to open a redundant font discussion here.
In principle, XeTeX works fine with .otf or .ttf font files. However, in some cases the .ttf version of a font lacks some metadata (for compatibility with older versions of Windows), so when both are available you should prefer the .otf.
An older font format was Type 1, which consists of a pair of a data file .pfa or .pfb and a metrics file .afm. While XeTeX has some support for Type 1 fonts, you will most likely find it deficient (no automatic support for character variants, no support for the seac operator); avoid the Type 1 format with XeTeX whenever possible.
The other types of files are of no interest to XeTeX. (.pfm files are just .afm files converted to a Windows-specific binary format. .enc, .map, .tfm, .vf are support files for traditional TeX/pdfTeX.)