Well, I think the best one would be from Jens Hensen since they're the ones who made the rings for the movie
Here's a link to their store:
http://www.jenshansen.com/
They do come at a hefty price though. If you want to save, I would recommend getting their silver sterling one and have it gold plated by them.
Also, Hecubus has started an interest thread for regular and Sauron-size replicas of the One Ring which will be made out of bronze and then gold plated.