Now this isn't which class you think is the best, this is the class you have the most fun with. Gunner. Discuss..
Assassin just because moving around like a crackhead while everyone else seems to be stuck in place is fun.
Personally, I like being a tank because I enjoy blowing up bots and melting moneyballs. Free juice off laser blazers is only the icing on the cake.
I honestly love playing Assassin, some of my most succesfull games where infact as assassin and you can really tear it up if you get going. Still, assassin is by far my least played class simply because it suffers to much from even only slight laggy games. But generally it's a really well designed class and to be truely succesfull requires some finesse and thinking ahead, love it.
The Assassin is a truly unique playstyle, and when it goes well you will feel rather elated in your pantaloons. Tank, I love you and your luxuriant beard, but good-times Assassin makes you feel like a sexy ninja.