In my opinion its gta 2, as if you ask me if i prefer gta san andreas or gta 4 ...
Of course the answer ( in rockstarīs case ) is evolution...
Each Gta game that comes out is better than the last so gta 2 was gta 1īs evolution and therefore is better 
But hey, just my opinion , 2 great classics indeed !!

Two great classics indeed, but i somewhat disagree with the statement i colored, since the day that IV got released for that matter, graphics and a new physics engine is hardly evolution, and even if it can be called that theres still the pretty hughe fallback in functionality/possibility's.
Bscly too me this is a remake of gta3 following game standards of nowadays (graphic & physics wise) and definatlly not anything of what i've hoped for, R* did a good job 'shattering dreams', good thing their lazyness (in essence still the same engine) will help us modders re implement all our beloved san andreas features (and more ofcourse)
Back to the classics, i liked them most of all gta's, and sometimes still play them, did you guys know you can get the pc versions of them for free, legally from Rockstar? (for a while btw, since sa release if im not mistaken)