You realize you'd have to edit the topic for a fair "best-GTA-city" trial,because a new Libert City is coming up this month ?
If there was an option for San Andreas as in 2004s game I'd choose that , but yes SA is rather a state.
If Vice City was bigger and had more expensive-looking buildings instead of for example some homeless people shack near the bikers' club, I'd choose it. But since it isn't I choose Los Santos , kinda looks like Vice City - mostly good weather , not unlike all-day rainy Liberty city in GTA3 or LCS.