Kyle Reese indicated that the T-800 was the state of the art in Terminator design:
The six hundred series had rubber skin. We nailed them easy. But the
eight hundreds are new. They look human. Sweat, bad breath,
everything. Touch it, you’d feel warmth. But by then you’d already be
dead. Very hard to spot.
Terminator: Frakes Novelisation
He was of course wrong. After sending its top of the line production model (and realising that it hadn't worked), Skynet also sent along a little something it had cooking in the lab:
"Not like me. A T-1000. Advanced prototype, A mimetic polyalloy."
Terminator 2: Judgement Day - Frakes novelisation
Note that James Cameron felt that sending the T-1000 was an act of extreme desperation by Skynet since the "learning mode" Terminator could easily turn against it:
"I started thinking about the film in two stages. In the first stage
the future sends back a mechanical guy, essentially what The
Terminator became, and the good guys send back their warrior. In the
end, the mechanical guy is destroyed. But up there in the future,
somewhere, they say, well, wait a minute, that didn't work; what else
do we have? And the answer is something terrible, something even
they're afraid of. Something they've created that they keep locked up,
hidden away in a box, something they're terrified to unleash because
even they don't know what the consequences will be - they being the
machines, now in charge of the future.
JamesCameronOnline - Terminator FAQ