Best Agency in Taiwan

Agents in the ESL world don't have the best reputation, and for the most part for good reason. They connect teachers with jobs and take a cut, whether directly from the teacher or from the school. A lot of agencies also get involved in other shady practices. If you use an agency, you're going to get paid less than if you find a good job on your own. There's no way around this.
So, why use an agency? It's actually very hard to sift the good schools from the bad. Working conditions can get pretty terrible on the bad side of things. If you're applying from your home country, it's virtually impossible to know what you're getting into. If you're already there and you do a great deal of research, you still won't really know until you get there. Reach to teach has been around for a while and they have longstanding relationships with a variety of schools.
As far as Agencies go, Reach to Teach is hands down the most responsible I have come across. They are very straightforward and they put a great deal of effort into finding the right job for each candidate. Reach also offers a strong social network and ongoing support to teachers once they arrive and throughout their year long contract. Whether or not you chose to use an agent or not is really up to you. If you use an agent, I strongly suggest you chose Reach to Teach.