Phantom: Requiem for the Phantom is a prime example of a show that is too overrated by fans, who easily overlook some gaping plot holes, and downgraded into oblivion by haters, who cannot see anything but those plot holes. I think this show sits somewhere in between: it is a fine show to watch, nothing exeptionally great, but not bad either. If you're trying to decide on watching this, let this be your decision after 1/2 episodes: do you like the character relationship of Ein and Zwei? Go on. If you do not, don't bother with the show. I will try to provide my arguments with as few spoilers as possible, but I cannot completely guarantee a spoiler-free review. Here goes.
== Story == 7
There are essentially two layers to this anime. On the superficial level, you have your standard mafia-esque corporations trying to gain more power, with the main corperation being Inferno. The distributions in power are a little vague in the beginning, but as the show goes on, one can see that higher-ups in the corperation are allowed a great deal of freedom when it comes to handling problems. Bosses will frequently go behind each other's back to solve personal problems. This would make for quite an interesting story of intrigue, but is a little dulled by the second layer of the show.
The second layers revolves around our two main protagonists, Ein and Zwei. The complex relationship that these two characters have makes for quite an interesting main character cast, but I felt its relationship overshadows the story of Inferno and its attempt at gaining more power in the underworld.
I found myself a little disappointed when the show shifted from the intriguing development of the main characters to highlight the structure and plans of Inferno. I understand that this is necessary for the plot to advance and therefore for the characters to even develop, but hope you also get what I mean by this. The story also suffers from an insanely large plothole, which I could not overlook and I will talk about in the Characters section.
== Characters == 7
The main characters is where the show truly shines, but is also a big downfall. Ein, being completely brainwashed now has to work together with Zwei, who is in the process of being completely brainwashed, but deep down is heavily conflicted. I loved how their relationship was set up, being purely platonic and bland at first, but slowly turning into something deeper that I will not explain to avoid spoilers. There were times that I completely loathed the decisions they made, which was proof for me that I was quite heavily invested in the main characters.
As with the story, the main characters overshadow the supporting characters, who are in this case mostly Inferno members. This ties into the more 'boring' superficial layer of the story. It was also nice to see that the show pulls off quite a few risky plot-twists, some of which work out greatly, while others fail horribly.
This leads me to a plot hole, so gapingly large that it almost swallows the entire show. This is also why many people are so put off by this show. About 75% in, there is a time skip of two years. Some characters haven't aged a day, while another turned from a child to an adolescent in a mere two years. Not only is this completely illogical, it was also not even necessary to keep the story going. The story and characters suffer immensely from this. Until the end of the show, I could not get over this. Your opinion on the show will most likely hinge on this specific time skip. I was really put off by this, but continued to watch anyway. The opening and ending also change after this time skip, which made this plot hole even more terribly obvious. Judge this how you will, but you cannot deny the irregularities that this time skip brought.
== Animation == 7
The animation is good. There is nothing wrong with it, but it isn't amazing either. Some characters, often ladies, are sometimes irregularly shaped (scenes with weirdly long legs or unevenly sized breasts), but the rest looks good. Characters look great, are nicely animated and some painting-like scenery fleshes them out well, while giving you that artsy feeling.
One thing bothered me though, and that has to do with the fan service. In the first minute of the first episode you will see a half-naked lady lying on the beach. BAM! Two tits in your face. While there is nothing wrong with that, I immediately thought: 'Oh it's going to be like that, huh?' But it wasn't. During the rest of the show, the animators went out of their way to not fully show nudity, to the point of unrealistic, nippleless, naked girls, which makes me wonder what the use of that very first scene was to begin with. You have the R+ rating already, so just show nudity when it is appropriate, or fully go out of your way to show nudity, and avoid naked ladies without nipples altogether.
== Sound == 8
The sound is great. The opening song sets the mood nicely, while tracks in the anime always fit the scenes. Some soundtracks are so great or are so obviously made to make you feel a particular emotion, that it can even feel out-of-place in some scenes. It is just too present and obvious sometimes, but most of the time augments the show perfectly. The second opening and ending songs are used after the time skip for the last few episodes, which made me feel weird, almost insulted. For me there was no reason why they should change the songs to make the time skip even more obnoxious.
== Enjoyment == 7
There were times when I was heavily invested in the main characters, only to be pulled out again by a dull side-story or hateful decisions some characters make. I still cannot fathom some of the plot twist inducing decisions that were being made and can ultimately also not say that I always enjoyed this show. A few times I just sat there, looking at my screen and asking: 'Why the fuck did you have to do that?' Sometimes the show dragged on for a bit too long on a specific subject or side-story, making me wonder if it had been better for the show if it had a few less episodes.
People have been comparing this show to Black Lagoon a lot, but I must warn you that apart from the mafia groups and the conflicts of the main protagonist, the mood, setting and goal of the shows are completely different. Black Lagoon is much more action-oriented, while Phantom focuses more on character development.
Ultimately, this is not a bad show. It is not amazing either. It's just decent. You will probably like it if you like the relationship of the main characters, but do not try to sell this as the greatest anime ever made, as its flaws are too obvious to ignore.