Why the rendering of some 2D textures are ugly ?

Hello, i have a big issue with the rendering of some textures off my 2d game.

All the graphics are well displayed with the right graphic quality except some textures that are displayed ugly. I use png with transparency. For quality setting, i tried DXTT5, DXT1, ARGB 32 bits compressions and they all render ugly. The thing incompréhensible is that in the texture inspector, the preview is correct but ingame it's ugly and opacity is not repscted. And the player settings quality are set on fantastic.

I can't find where the problem is. Please help me.