An anime that is really close to phantom?
 

As the title says,

Im looking for an anime that is really close to phantom:reqiuem for the phantom: ( don't mention darker than black.)

What I actualy want is an anime with thriller/mystery/romance/(seinen) and must be almost same as phantom.

Same voilence
(almost)same drama

Greetings,,

Somewhat black cat

Noir comes to mind since it's also by bee train.

Monster also comes to mind

And I know it might not seem like it but Madoka Magica qualifies as a thriller/mystery/seinen,it's also written by the same writer as phantom.

Rainbow: Nisha Rokubou no Shichinin, can't go wrong here. Even though anime is not about assasins it still gives the same vibe as Phantom.
I adore Phantom series and Rainbow is the closest I have found to it, my signature is from that anime.
It has everything you asked: Some thriller and suspence, slight mysteries, a bit of romance... everything you can wish for. It is a must watch.