The longest manga series could be considered 'Golgo 13'. It has persisted for a long time because of its unique storyline and dedicated fan base. Also, its consistent release schedule has contributed to its longevity.
The title of the longest manga series is often debated. 'Golgo 13' is considered long-running, as it has been published for decades and has a huge number of chapters. The longevity could be attributed to its wide appeal and diverse storylines.
I'd recommend 'Naruto'. It has a lot of action and emotional depth. The characters' growth and the plot twists make it very compelling. Also, it's easy to get into and follow along.