I think its about the atmosphere and characterization. Vice City and IV had a great story and believable characters and so the player gets in the mood easily. In San Andreas the story is too stretched and feels disjointed. GTA V feels like its trying too hard to be "crazy" (f*cking Trevor) and fun while selling short because of HD era realism somehow...the game feels like a confused project. Perhaps R* tried to make everyone happy, and failed, because greed.
I still like them all, except VCS, LCS and CTW, I really don't see the appeal guys. The games were released because they knew you'd buy them. The story and characterization of the games was lazy and boring.
And to the guys saying VCS had the perfect mix...the property development missions were boring/repetitive and the last mission was ridiculous and aimed towards the drooling idiot.