X265 seems to be the next big thing but when I try to watch a movie encoded in X265, either with Kodi or VLC, the result is always worst than with the traditional 264.

Anybody has a solution ?

https://en.wikipedia.org/wiki/X265

Try using MPC - HC 64 version 1.7.1 designed for use with H265 movies and tv. VLC will work with it but sometimes buffers.

Also check the resolution. Most h264 encoded videos are 720p, whereas a lot of newer videos in x265 are 1080p. In that case it's a lot of extra decoding to do for your computer and it may just be too slow for the job.