Formal:
* J'aime beaucoup votre robe. (I like your dress very much)
* Votre robe est très jolie. (Your dress is very pretty)
* Je trouve votre robe très élégante. (I find your dress very elegant)
Informal:
* J'aime ta robe. (I like your dress)
* Ta robe est super ! (Your dress is super!)
* J'adore ta robe ! (I love your dress!)
More specific compliments:
* J'aime la couleur de ta robe. (I like the color of your dress)
* La coupe de ta robe est magnifique. (The cut of your dress is magnificent)
* Le tissu de ta robe est vraiment beau. (The fabric of your dress is really beautiful)
Choose the phrase that best fits the situation and your level of familiarity with the person you're speaking to.