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Avatar is the film to definitely test out blu ray if you are new to it to show what it truly offers, Avatar looks absolutely astonishing on blu ray.
Tron 2: Legacy is beautiful to look at also.
The animations from Pixar, WALL.E for example, look amazing.
Coraline is delightful if you like things like A Nightmare Before Christmas/darker animations, the main characters jumper/top in one shot looks stunning in the detail department.
I haven't seen it on blu-ray, but Batman: Dark Knight is suppose to be excellent.
If that's what you mean by best, the quality of the blu-ray visuals.