Giving compliments to a female friend or girlfriend would certainly make her day. The expressions below are essential the same but said differently depending on her relationship with you. For your girlfriend, you can say the following:
For a normal female friend, you can say the following:
Xx san wa kirei desune.
Note that for (1), you are using casual speech, which shows closeness between you and your girlfriend. For (2), you are using polite speech, which shows respect for a normal friend.