Out of all of the GTA's I've played this is my score
Maps
GTA III: 5
GTA Vice City: 8
GTA SA: 9
GTA IV: 7
GTA III looked and seemed big but it was small, Vice City's was fun and is like a joy ride, SA's is huge and has different zoning areas that are lacked in most GTA games, IV was huge but a bit more explorable that others, if you knew how to.
Vehicles
GTA III: 7
GTA Vice City: 8
GTA SA: 5
GTA IV: 8
GTA III's cars had great models and handling unlike most games and it sounded like you were flooring it... since its that or idle or reverse, GTA Vice City's cars were exotic and fast but also had many slow and powerful trucks and regular cars. GTA SA's had poor and easily damageable cars, it was more of humor than realistic, but had a nice variety of bikes and trains , GTA IV's were fast and are the first to have different (easily noticed) throttles instead of the regular FLOOR IT they all had many paint schemes and some even different versions of the car and a new 3D damage model.
Enviornment
GTA III: 3
GTA Vice City: 8
GTA SA: 10
GTA IV: 9
GTA III, turn off your lights and set brightness on max, lighting is terrible in GTA III, I squint all the time to figure out if its an outline of what ever it was. Vice city, had great lighting, you could see everything and wonder in to stuff and have fun. GTA SA, realistic fog and storms, real lighting instead of green overlay. GTA IV, at certain times taillights and headlight are huge big blobs of red and white, very annoying, objects are well lighted and colored like in Vice City.
Campaign
GTA III: 3
GTA Vice City: 7
GTA SA: 6
GTA IV: 8
GTA III, WTF uh... none of it made sense and I lost interest and decided to just steal cars and kill people.... GTA Vice City: I laughed the entire time and rocked out to 80's Metal \m/. GTA SA: It waz kewl yo, had its ups and downs but it was ok. GTA IV: I was honestly really interested in IV's campaign, keep me on the edge of me seat doing cool stuff and laughing at the Bellics do "sheeeet"
Graphics
GTA III: 3
GTA Vice City: 7
GTA SA: 8
GTA IV: 9
Overall
GTA III: 4
GTA Vice City: 7.5
GTA SA: 7.5
GTA IV: 8
GTA III, fun but eh... no . GTA Vice City, personal favorite, GTA SA, really good, GTA: IV, still new so lets wait a while.
Just my comparison of the games, thought it would help out.