Depending on what type if assassin you are: Range or up close.
For range, I would choose Jenassa, the Dunmer mercenary in the Drunken Huntsman. She has a morality of 0 (so she can murder for you), she is good at sneaking and archery AND she is the ONLY follower that can dual wield. She won't get in your way much and attack only when you are spotted. BUT, she is not that good at sneak attacking with a dagger. She is a good back-up as an archer and can take you out of tight spot, mostly if you are an hybrid and uses spells. In melee, give her 2 swords and a good armor with resist magic capped and she is lethal. If you are an archer assassin, big chances are that she will be MUCH better than you at melee. She levels only to 40, but at that point she would have Maxed out Archery and Sneaking, and nearly 1 handed weapon too, with 75 in light armor.
For Up close, I would choose Cicero or the Brotherhood Initiate. You can actually have BOTH at the same time, due to a bug with the Brotherhood Initiate. They are VERY good at sneaking and can assassinate up close pretty efficiently. In melee, they are good, but they cannot DW like Jenassa does (there is a bug there). They are not that good in archery, as far as I saw it. Aela or Jenassa are better for that purpose alone. Other than that the DBI is the ONLY one that will level with you till 81. Cicero goes to 50.
That said, if you are a real assassin, I don't see the need of a follower outside for the pack-mule purpose. Assassin are usually lone wolf and works better alone.