I think they are all great, but which is best depends on which aspect of the game were' talking about:
For sheer novelty and genius, especially for the time: The Sims 1 is best
For scope and quality of active gameplay, and the ability customize and have control of every aspect and sim in the game: The Sims 2 is best
For best world design and variety of worlds and exploration of Worlds: The Sims 3 is best
For artistic style, ability to customize Sims (CAS), ease of building, smoothness of animation and overall expressiveness of Sims in gameplay: The Sims 4 is best
Sims 4 is also best in terms of the actual quality of CAS and BB items provided by the devs, but both TS2 and TS3 did a better job of letting you customize style of said items.
Each has their own merits...a dream Sims game for me would be if they could combine the best of each into one Super-Sims game.