As far as I know there are currently no rabbis who are actively involved in legislating rabbinc-level laws. Those laws were set in stone centuries ago. As DoubleAA mentioned in the comments on your question, rabbinic laws could theoretically be repealed by a court under certain circumstances; however, there is not currently a court as great as the Sanhedrin in order to do this.

As far as modern decision-making goes, current rabbis are not determining new laws. Modern rabbis simply interpret the laws that have been in place for a long time in a modern context. There are a few great rabbis who answer the most difficult questions; however, most scenarios that you will encounter in a day-to-day context are unlikely to be so complex that such a great rabbi must be contacted. Usually your local Orthodox rabbi will be sufficient to tell you what you should do in a given circumstance. He will not be creating the law; he will just be telling you the law that is already on the books, and he doesn't have the power to change it.