Replaying and beating GTA III, Vice City recently, and playing a lot of San Andreas, IV, V, and RDR2 recently, Vice City is my favorite GTA by a long shot.
You have freedom regarding how you reach the ending. The businesses you buy, and more importantly the order, combined with smart saving (not buying or needing weapons/ammo from stores), along with getting bonuses from side activities, influences your creativity and decision making in story missions.
The soundtrack, environmental design, mood, humor, use of color, absolutely everything on point. Story has no filler. Being a psychopath business mogul in Miami is an amazing open world power fantasy.
GTA VI has a lot of drug related mechanics most likely. Being a drug dealing kingpin in the modern American nightmare known as Florida, with beaches, islands, and swamps, sounds baller.