Rules:
Data has a phaser
Terminator Arnold has a Ar10 with m203 grenade launcher and a shotgun
Rnd 1: with weapons and morality turned on for both
Rnd 2: with weapons morality turned off
Rnd 3: hand to hand combat morality turned off
Who wins in their silver prime?
Most civilians vastly overestimate the power of grenades (of all types).
Here's a video showing what an M203 can really do:
We already know that Data is immune to hand held projectile weapons, and I expect he would easily be immune to even a direct hit from an M203 as well. Considering how vastly more powerful a phaser is, I would easily give the win to Data for the weapon rounds.
As for hand to hand, Data has been shown to be fast (40+ MPH) and extremely nimble (far more than a human) and canon says he has mastered all fighting/combat styles. The terminators have not been shown to be anywhere near as fast or nimble (T800 burst speed up to 22mph). As for strength, Data was shown to easily bend a sci-fi metal bar at 40 kilobars (~580,000 PSI) far more than enough force to literally crush graphite into diamond in his hand. We see that the Terminator is eliminated by a hydraulic press in the first movie, canon suggests it was a 1,000 ton unit and divided by the size of it's upper torso (about 24" x 36") that works out to around 2,600 PSI (0.18 kilobar). We also see that they can lose limbs and be torn apart in fairly simple scenarios. As seen on screen, terminators can be heavily damaged/destroyed by pipe bombs, 50 caliber weapons, even damaged with pry-bars etc.
Based on this, I don't think a T800 type terminator would stand the slightest chance of winning vs Data. He could easily outpace and outmaneuver a terminator and literally very easily tear it apart. With the demonstrated (not even maximum) power Data has, he could literally crumple a T800 into a ball as easily as we crumple a sheet of paper.