Best talent agency to join?

Each agent will allow you to get to every single gig eventually. They do however, change how frequently those auditions appear. For example. G.R.A.N. will cause period piece dramas to occur more, like Of Tea & Treachery. While Everyday Extras will a cause more commercials and lower end gigs to appear. Well Suited will even get you access to higher end movies before normal, allowing you to risk it big when you might not be skilled on it.

A few other perks
A.I. Agency - Awesome daily notification reminders. (Sure, they are awesome, right?)
G.R.A.N. Agency - Flowers when you do well. Cookies when you fail (sometimes)
Well Suited - Slight pay bump.

There are more differences, but I'll let you discover them.