Personally, I say the best in the series...
GTA SA offers up a lot of different cars. It is set around the same time as GTA III (I think), and it offers stuff like regular family cars, trucks, motorcycles, and then the stuff like semi trucks, muscle cars, and of course, you get the really fast, super accelerating cars that look so cool. But then there are some different vehicals, such as the Dune Buggy, and also little hot rod like dune buggy's that remind me of Smugglers Run.
Another interesting feature in this game is the ability to customize your cars. You can find little garages that wil let you do a lot to your cars. You can change the color, change car parts, such as rims and bumpers...but the best part is the ability to fit your car with noss, giving you the ability to give your car a super-boost...the down side is that these cars you pump so much money into just get wrecked, and if they blow up your hard earned cash is out the window.
The weapons system remains the same as in GTA VC, in which you can only carry one type of each weapon, so only one melee weapon, one heavy weapon, ect. GTA SA offers up lots of different weapons, such as pistols, rocket launchers, sniper rifles, machine guns, ect.
An interesting addition to this game is the stats. You start off with no stats, but as you do certain things you increase them. Your stats include Speed, Strength, Stamina, and even Lung Capacity (you can now swim in this game.) It is important to work these up, because they will be of use when you are playing the game...but if you don't use the skills you work up, such as Strength, it will decrease, and then you have to work it up again. There are also stats for the weapons. As you use pistol type weapons, you will eventually level them up, making you more accurate with the gun. This helps for when you are using guns in missions. Also it does this with vehicals. As you drive regular vehicals (4 wheeled ones) you will increase your handling, as you drive motorcycles (any two wheeled vehicals) you will increase your mototcycle handling, and as you fly air planes, your flight handling will increase.
A great addition to the series, and I reccomend picking this one up.