Hello, I'm looking for intresting anime and manga about assassination.
The reason is mostly because I watched Phantom: requiem for the phantom and got hooked 😛
If you have any recommendations please let me know 😀

M-chan

Dendrobates is a pretty classic example. The protagonist has no qualms about his job either, which is always a plus.

Freesia is similar. Though the MC isn't very likeable in this one, it's arguably more realistic.

Other classic assassin/hitman stories:
Gunslinger Girl
Ordinary ±
New York no Benkei
Deathco
Number
Number Five

With a supernatural bent:
Luck Stealer
Andromalius
Arachnid

Ansatsu Kyoushitsu - Not in the typical fashion; more like assassination training than the professional variety.

Funouhan, maybe, but he's no ordinary killer: basically a psychological torturer who accepts hits just so that he can enjoy his hobby.

Akuma no Riddle - If you don't mind mild GL.

Akumetsu - If a vigilante who kills corrupt politicians of his own volition counts as an assassin, anyway.

Girl May Kill - Iirc, this mostly explores the psychological aspect rather than the actual killing.

12 Nin no Yasashii Koroshiya - Leo Murder Case -- Focuses more on one particular murder than on assassination as a whole, despite it being the MC's job.

Kimi no knife, definitely a must read

If you don't mind a slightly comedic one, try Alyosha. 😀

There's also Cynthia The Mission. Which is more action based. It has a bit of gore though.

There's Anonymous which is a bit short.

I have a few more at the top of my head that I don't remember the name for at the moment.

Murciélago
Dokuhime... well, sort of anyway...
Akame ga Kiru!
Kure-nai
Vampire to Shinigami - Oneshot

And sort of surprised it hasn't been mentioned yet - Golgo 13

~4400 manga completed
~1700 ongoing...

Thank you, I'll try them out.
And by the way I don't mind GL, the only genre I'm not into much is horror 🙁

M-chan