Wow and Ouch! You were not kidding when you said you needed a tie breaker. Well in my personal opinion on these three anime. Kinshin or Samurai X would be a very short fan fic. The first time Kinshin came across your OC assassin. He/she would be dead. You have to remember here. Kinshin is a MASTER, and your OC assassin cannot top him. Samurai Champloo. EH! It would be alright, but Mugen is a CRAZY F**ker with a sword and Jin is master swordsmen. It would make for a great fight, but is it really their style to get involved in something like that? For them to get involved the assassin would have to be coming after one of them.

Bebop! There is the winner. Spike against an assassin. OMG! I can't think of a better fan fic.

I will break this tie. Cowboy Bebop is my vote.

Good luck Kita, and TheO Story writers. Can't wait to read the new story.