Hi

Do you make only this trip or other trips in Europe?

The best route would be Milano - Zürich - Basel SBB - Strasbourg - Lille. You could find a direct train from Milano to Zürich and a direct TGV from Strasbourg to Lille and avoid the bus to Geneva and the change of station with the metro in Paris.

When exactly would you like to travel? There could be some engineering works on a certain section. 

Just to be sure: what pass do you have?

I agree with @Danhiel recommended route above, but rest assured that on such a long journey there are alternatives. You might even travel via Germany and Belgium, should the Strasbourg-stopover not be possible.

And as this is a forum for people travelling by train, of course we all agree that you better avoid buses.