The best assassin that i have found is the Khajitt. They are perfect because the start with +10 to acrobat,
+5 to Security and Sneak, +5 to light armor and +5 to blade. Of course if ur one of those crazy gook players who like hand to hand, they start with +10 to hand to hand. (all this info is in the basic instructions booklet) I am just lettin u know
. If ur in the Dark Brotherhood take Vincente Valtiere up on his vamp offer and u get some crazy good stats to add on. (vampires can be slightly anoyin to use cause of the whole "u cant wait for a day or your crispin in the sun" deal) but I think its well worth it. Also consider using an Argonian. They start with good stats for an assassin. One last tidpit of information for u. Consider also adding Alchemy to ur major skill list. It will allow u to make good invisible and detect life pots. Beside that u can also make powerful poisons to make ur opponent easier ( only poisons i have found that work good are paralyze and damage health). Alchemy is also easy way to make money!
Give it a try and see what u think!
P.S.
Dont make an Imperial Assassin, they no good for anything!
Spill some blood for me brother!
-Vincente Valtiere