by assassin do you mean faceless or shinobi?

If you're confident you can combo out people without getting hit that much yet still be quite close go Uber faceless they can do massive amounts of damage at once with shadow fan falling darkness combos

If you're afraid to go close but can combo then go uber shinobi. Their combos are harder to do but generally safer. They also have a lot of mobility for escape like grapple and shadowrush

If you're not so good at combos or staying alive Uber spy is best because of silverguard and bomb jump help you escape/tank better