Android Wear, while still in its teens, is perhaps the most full featured OS as far as the smartwatches go. Yes, we are aware of Apple Watch, but since that is not out yet and nobody seems to have much clue, we will reserve our judgement till it is, and call Android Wear the champion. You have about 4 models of Android Wear that you can currently pick up and few more are in works. Right from typical round to sluggish square watch faces, there are options galore in an Android Wear out of the box. To help you take this experience a couple of notches further up, FaceRepo is an interesting option.
FaceRepo, as the name suggests is a repository of custom watch faces for Android Wear as submitted by Android Wear lovers on the web. You can download the zip file of the watchface of your choice depending upon the watch and then go ahead and open the Face using the file browser that is built in to the Facer application that is available for free on Google Play.
You do have options to rate, comment as well as share the watchfaces, right from the repo itself and can also make a submission to help out the community. It is slick and interesting way of keeping your watch fresh, and obviously the best thing about it all is that the service is community driven so you are likely to see a lot of new add ons over the period of time.