There is no "best" service. People do it competently or they don't. I can give you some personal recommendations or you can also send in a request to support.
Trademark is a tricky business. It is possible you may not be able to trademark your domain name and best if you speak to a reputable attorney who seems to have done this for a reasonable period of time. Otherwise you'll spend money needlessly on fees (It's $325 or more to register plus other costs you may incur.) Best of luck, let us know if you need help.