Guild Wars 2

I want to craft the melee back item in the mystic forge for my guardian but i cant find which one i need to craft for my class.

I need tohe pure dps one with power pre and crit dmg.

You want either http://wiki.guildwars2.com/wiki/Quiver_of_Swift_Flight or http://wiki.guildwars2.com/wiki/Book_of_Secrets. If you do alot of FotM than you could also use a http://wiki.guildwars2.com/wiki/Prototype_Fractal_Capacitor_%28Ascended%29

Which one has the 5% crit damage ?

Both.

http://wiki.guildwars2.com/wiki/List_of_ascended_equipment

Quivers have offensive infusion slots, books have defensive infusion slots. Other than that, those two have the same stats.

Yeah there are three different Backpieces that have the exact same stats. As Mad said the quiver has an offensive infusion slot and the book has a defensive one. Depending on what material you use to ascend the fractal capacitor will determine if it has an offensive or defensive slot.