I love Nocturne to bits and he's still really good, but right now my first recommendation for the assassin role would be Elise. She's way easier to do well with nowadays and she's as resistant to falling behind as one can get without being a straight support jungler like Nunu or Taric.
Plus she can go mid or top too, so you got versatility as well. --- "ˇViva La Revolución!" -Salvador Limones