Well, the first GTA game that launched the series was actually GTA III. It revolutionised the world of games. Starting with GTA III: In GTA III, basically the main theme is to get revenge and do anything to get it, which includes betraying your own friends, killing police officers and unimaginable things such as RPGing a plane with a drug package. GTA 3 was one of the most popular games released that year.
In GTA Vice City, its basically the same, only with a few extra features such as changing clothes, talking etc. The theme is also similiar as it involves taking revenge on Sonny Forelli, Tommy's boss who set him up and got him in jail for 11 years. Vice City was also extremely popular with some controversy over Haitans and Cuban portrayals.
GTA San Andreas was rated one of the best ps2 games in history. It invlolved even more features such as Weapon skill, respect, gang recruitment, sex appeal (for luck with the ladies!), fat, muscle and much more. This game far outperforms the previous games with better graphics, better detailed gameplay and much more features. It, however, was met with much controversy when the "Hot Coffee Minigame" was revealed to be in the games network. A patch was released and Rockstar Games was fined a large sum of money.
GTA Liberty Stores ,a prequel to GTA 3, proved to be a disappointment with hardly any of the features from GTA San Andrea. There are no new features included in this game. This game was rated 80%-90% by game critics, a far disappointment from other games. The only good thing about this game was probably cooler cars.
GTA Vice City Stories, a prequel to GTA Vice City, had added a huge new feature which included owning your own businesses and getting paid at 16:00 every day. The gang recruitment feature was brought back and VCS includes an appearance of Phil Collins and a concert of him.
GTA IV made huge sales on the first day of release. It was ranked #2 game OUT OF ALL GAMES. It included many more new features but however did not include some features from the older games. IV is extremely realistic. Missions have different endings such as killing or letting Darko Brevic live and gameplay is extremely realistic. Just jump out of a moving car or smash your motorbike at a high speed, BAM it's extremely fun to watch Niko get owned by diving into incoming cars or falling from high heights.
Overall, I'd say that GTA IV is the best out of all the games with GTA San Andreas following close behind. However GTA San Andreas was only on PS2 and PC while GTA IV required high version consoles such as PS3 and Xbox 360. So its like this: Who is stronger? A 10 year old who can lift 50kg or an 18 year old who can lift 100kg? Of course, the 18 year old is stronger but the 10 eyar old was not as developed as the 18 year old and would require more work.