Katekyo Hitman Reborn is a terrible shonen series that is very badly written and doesn't reward its viewers with an actual conclusion (even the manga ending is stupid).
Reborn is baby assassin hired to raise Tsuna, a whiny wimp who was chosen randomly for no actual reason, to be the next mafia boss for the vongola family in Italy. From the beginning of the show to it's end (and even the end of the manga) Tsuna refuses to become the vongola boss because he is a coward, even after saving the world multiple times he still refuses for no valid reason. It's not like he has any other goal in life, he showed no passion towards anything which leads us to believe that he's a lazy deadbeat throughout the whole anime without actually developing as a character. The only times he becomes inexplicably calm is when he turns on his powers and turns into a different personality to take on his enemies. That completely stops him from growing as a character because the Tsuna that fights is different than the normal one and thus he can pass through every arc without actually growing a pair and taking control of his future. Nothing on the mafia is delivered throughout the story, in fact if the word mafia didn't get repeated over and over you wouldn't guess that that's what this anime is about. Nothing about organized crime, nothing about gang wars, and nothing about drugs or police. The worldbuilding in this anime is horrid, it feels like there's no one in the world other than the established characters.
1- 20 or so episodes of comedy and character introductions. Most people believe that these are the worst episodes of the show, but they aren't actually worse than what's to come. Yeah the humor is flat and it's lacking in action but it doesn't fill itself with plot holes and obnoxious characters while also not making you lose all hope for the anime yet as you know that there's action to come.
2- Mukuro arc. First battle arc, forgettable.
3- Varia arc. The best arc despite having many flaws. Old villain becomes ally for no reason. Random high school boy with no experience sword fighting beats an experienced assassin after a week of training, another high school boy obliterates a killer robot by just hitting it with a stick.
4- Future arc. If the anime ended before this point I might've given it a 4-5 but this arc makes this mediocre anime terrible. After Tsuna and co. beat a small group within vongola they go 10 years in the future to fight a mafia boss who have conquered the world and beat their 10 years older selves. Power ups are given randomly to the heroes and for some random reason they can beat foes their future selves couldn't. It's like sending kid goku to the future to beat beerus, it doesn't make any sense. Add onto that the fact that powers became dumber now. Every person has a magical colored fire that they can insert into their magical boxes to summon a magical animal that allows them to use a random magical ability that they will use only once and then when they get into another fight they will get their next random magical power.
The manga continues after the previous arc and it still doesn't make sense how Tsuna and co. are struggling to beat anyone after coming back from the future where they defeated the man who conquered the world.
Characters are very flat in this anime I'm confused why anyone likes any of them. Tsuna is a coward that never grows up to give any satisfaction for sticking around to see him change. Hayato is a Tsuna worshipper. Takeshi is always cheerful and Ryouhei is hot-headed and manly I guess. Lambo is an annoying brat who got chosen as a member in vongola for no actual reason. Hibari is op from the start for no reason at all and has no personality. Mukuro has no reason to join vongola and Chrome is cute I guess but that's all there is to her. There are two girls who are supposed to be love interests but nothing on that is ever delivered throughout the anime or the manga for that matter so all they ever actually did was stand around and repeat Tsuna-kun over and over.