Probably San Andreas. When it comes to overall gameplay, GTA IV was best in my opinion. The detail put into that game was amazing. GTA SA is the most memorable because of my childhood PS2 days. Also, out of all the GTA storylines, San Andreas was the most well throughout. CJ's past was way more relatable than Nico Bellic (IV), Micheal, Franklin, and Trevor (V). I felt more connected to him as the story went on. In GTA IV, you become a hitman, complete missions but not really achieving gratitude or vengeance. Only to end up losing the love of his life. Then finally realizing revenge isn't the answer.
On a side note, it would really be nice if Rockstar Games remade San Andreas. I'm tired of playing HD modded versions.