If the batter hits a single to left field and a runner is thrown out at the plate is the batter credited with a hit or fielder's choice?

This would be a hit. The only scenerio that this would be a fielder's choice is if the bases were loaded during the hit and the runner that was thrown out was on third base during the hit.

A fielder's choice is scored only if the batter would have been out had the fielder chosen to go after him instead of a runner already on base. If the batter would have reached first anyway (such as in the scenario described here), he would get a hit.