Perhaps the most important part of your relationship with an agency is the recruiter. Pick the recruiter you "click" with. Forget about the agency brand and BS. You can also largely ignore online reviews as being agency reviews, they are more about the recruiter relationship. Just as no one can predict who you click with in real life until you spend time with them, so it is true here. So just call a number of agencies and talk to recruiters. Besides finding those that are comfortable to work with, you will learn a lot about travel as you sift and collate their responses to your questions.
I would suggest finding a several good recruiters (agencies) and establishing a business relationship with them (fully signed up). Even more than a full time job, traveling really requires having a Plan B and Plan C in reserve in case your first choice doesn't work out.