As a standalone sci-fi show, it could be a 6 out of ten however, the show is full of lazy tropes, cheap payoffs and bad writing to name a few and this is only the first episode. now some of the positives, the action and violence and cinematography are great to see and sets it apart from a lot of generic sci-fi but if all you want ok - good CGI and action then you might enjoy this show.
Now the story, this is the worst part about this episode as it tries to establish "new" lore too quickly for instance setting up the UNSC as the bad guy in the first episode which could have been an interesting topic if left it to the season finale. another sad topic is the chief is not the same chief as the games. In this series, the chief is seen as a panicked, whimsical and emotional man unlike the books and games where he is stoic and often referred to robotic and lacking emotion which works great as each time the chief shows some emotion in the games it has a huge payoff as the players understand the character more.
overall as a "halo" series it gets a 3/10 as the story is not halo and compared to many series the writing, plot and storytelling is very subpar for something that has been in development for nearly 9 years.