Because he is a very quick witted person(well God) who honestly wants vengance for all what his brother did to him in their younger years. It was a selfish purposes but if he wasn't as relatble a lot of people might not have liked him as much.
The reason Loki could be perceived as the "best" villain in the Marvel universe is that he always knows what to do. In any situation he has proven to have a back up plan. Loki is a very smart individual, and would most likely outsmart any others in war. He is also an extremely determined fellow. In Avengers infinity war, the opening scene, he tried multiple times to foil Thanos' goal. He may have failed every single time but it shows that he will try to get what he wants until that thing is gone.
