Many argumentative forms of writing begin with sentences like " I believe" or simply state things as facts.Remember to keep it personalized to an extent and have data to back it up so its also persuasive. For an argumentative thesis to the prompt “There’s a wide variety of human personalities exhibited by the animals in Animal Farm” you can pick a specific animal you thought showed those traits a bit more clearly. If you're looking for a counterpoint of view to the prompt you may try to step into the shoes of someone who does not believe the animals are relatable at all. Again i do not know the grading system you have,or the details of the assignment but i hope this helps. Have a great rest of your day!
~jet