Fighter.
No, really. Ok, it depends on what sort of assassin you're aiming for. Do you mean a guy who kills people for money, or a guy who sneaks around and shoves daggers into people's precious organs when they're not looking? If the former, I've got a nice Warforged Fighter (Dungeoncrasher) who walks around in adamantine full plate, smashing victims through walls and using the resulting hole as an escape route. He relies on the fact he's good at disguise and bluff, and nobody expects him to be an assassin.
If you mean the latter, I have nothing I can offer that hasn't already been said. Still, if the DM's being restrictive in classes, sometimes it helps to diversify your ideas of what defines the character. Of course, you should always play what you want to play, I just felt like chiming in, mostly.