Aristotle has many great accomplishments to his name in many fields of study. There are so many in fact I cannot enumerate all of them with a good level of depth, so I will instead focus on his biggest contributions. First Aristotle invented the field of logic. He several different books on how to make formal arguments while he was attending Plato's school in Athens. Before these books there was never a formal branch of study devoted to how to make sound arguments, so his books essentially started that. This work was aided by his mentor and colleague Plato. This is a major accomplishment if his time
Aristotle also made significant contributions to the field of philosophy. He made many contributions to this field that were initially very heavily influenced by his mentor Plato. Reflecting many of the same thoughts and ideas showing how closely intertwined their thought processes were near the beginning. We see some of his writing at the time talking about ideas like how men were given a divine intellect to be able to reason and understand the world. Though his later works seem to go against the ideas of his predecessor. This was a major accomplishment of Aristotle.
He also created the idea of academic disciplines. Prior to his work people like Plato and Socrates would write very elegant papers that crossed many different branches of study with one central idea tying them together. Aristotle didn't really write in this manner. He wrote in a very blunt style and categorized the different ideas that he studied going exceptionally in-depth into one with each writing. He created 3 branched of study. Productive studies, fields with an end goal in mind like engineering or architecture to build a road or battle strategy and Rhetoric to gain victory. Practical studies, fields with a goal of teaching people how to act like ethics and politics, then there were theoretical studies like physics and math which were fields for the sake of knowledge. This system of dividing up categories of knowledge is key to modern academia making this a significant achievement.
Another significant achievement of Aristotle, probably his greatest accomplishment is his work in zoology. When he left the academy of Plato after his mentor's death he went on to study the natural world and categorize animals. He documented animals’ habitats, migration habits, anatomy diet etc. for over 500 different species going into in-depth description of every animal. He also predicted many biological phenomena that were not proven for centuries from his research alone, for example embryonic development. He did this in the true modern scientific spirit before sciences were truly rigorous. He would make predictions and see what happened if he was wrong he would change his perspective and make his model fit the data not the other way around. Some of his discoveries were not surpassed until the 19th century. This is his greatest accomplishment creating many new discoveries, but also creating a model for scientific inquiry and discovery.