The model Arnie played in Terminator 2 simply because the director (JC) developed better dialogue and overall brought out the best in Arnold for the film. Although physically the model in Terminator 1 was far superior and in T3 there was potential for a classic and perhaps even best model but it was never used to full advantage.
The most realistic interpretation is the first movie. The Terminator in that movie is purely logical and everything he does, despite being cool, is for a reason, rather then just for the audience. The sunglasses, for example, aren't just to "look cool." He actually tries to maintain his disguise playing into the whole "infilitrator unit" role. He also kills the most people and is a total badass.
Yes, depends what one is looking for in a Terminator. The first one was brilliant, indefatigable, logical, almost seeming like a force of nature sometimes, and his purpose was well portrayed. The second one however was a different kind of character, and it worked well. Depending on how I feel either the first or second.
if i had to really decided i would say that THE TERMINATOR had the best t800. why? because like people have stated above. it had only 1 mission in mind, to kill sarah connor. and whatever came in its way was to be terminated.. now for example. in the terminator the t800 kills the guy who was giving him some rifles. and in t3 he did that lameas$ line.. TALK TO THE HAND... t3 definately was played to the audiences. in t1 the t800 kills the people who he wants clothes from. and in t3 he just takes the clothes away( granted he chokes that guy.. but still you know thats not what a terminator is like), and then that lame sunglasses thing. it was clearly seen that jonathan mostow wanted to make the terminator look friendly to the audiences.. crap