English Conversation Schools /Eikaiwa
The English conversation schools exploit Japanese insecurities about English to make money. Their teachers rarely have any teaching certifications, and the turnover rate is high. There are also few opportunities to study Japanese while working in such an environment.
Of the mainstream English conversation schools, ECC is probably the best, with the most generous vacation, salary, and training benefits.
AEON is also not too bad. They require working longer hours, but the company is somewhat serious about teaching. They have a big focus on children, so there are also some opportunities in that area.
The main GEOS website has been down for over one year (!) now, but they are still in operation. With that kind of sloppiness, though, I can't recommend them.
Gaba desperately tries to avoid giving anyone a full-time position; this is how they cut costs. They have positions in all of the big cities, but they generally don't offer visa sponsorship. I would recommend avoiding them as well.
Berlitz may be the most prestigious school, but I generally hear nothing but complaints from people who work there. Primarily this concerns the "Berlitz method" and the draconian way that they force teachers to fit a model that may not, in the end, be all that effective. However, the pay is decent.
James English School
James English School is a regional school (Tohoku only) but they're the best of the bunch. I have only heard good things from friends who worked there. The benefits are good, and the teaching environment is excellent. Of course, if you're not serious about teaching English, this would be the wrong school to apply. They expect a multiple year commitment.