Grrr...I can't really vote fairly on the poll. Best how? Each game had features which could potentially make it "the best". Here's my thoughts & opinions
GT2: best for just all around fun. Getting in some crappy car you've never heard of, and then changing it around to become a superstar. That's GT2 for me.
GT5: best for physics, car dynamics.
GT1: best for technical data. Power & gearing graphs still haven't been replicated as well as they were in the first GT.
GT2: best race mods
GT5: best online capability & fun (duh).
GT2 & GT4: best for easy money
GT5: best for atmospheric effects (duh again).
GT4: best understeer
GT2: best memories (GT1 runs a close second place here).
GT2 & GT5: best drifting capabilities (but in much different ways).
GT2 & GT4: best "encyclopedia" of cars (GT5 doesn't really count yet since most of the cars in this game are ported, not really unique to the game, but GT5 runs a close second since they FINALLY included my Boss Mustang).
GT1: best racing game on consoles (for its time)
GT1: best (fastest) menu system
GT5: best (most detailed) menu system
....I could go on but you get the picture. GT3 is the only one I haven't mentioned so far. That's because it's my least favorite.

OOPs!
GT3: best replay system. There I said something nice about GT3. 👍