Who are some of the best writers in the history of civilization? Who should I read for the purpose of getting better at writing?

The answers to these two questions are likely to be different. I think Nabokov is an incredible writer, but I wouldn't recommend emulating his style as a route to becoming a better writer. Still, if you're looking to read a masterful prose stylist, I don't think you can do better than Nabokov. Here's an excerpt from Lolita to give you a sense (although you should be aware that the narrator is supposed to be a pretentious pedant, so that's reflected in the writing).

Cormac McCarthy is another excellent writer, of a very different kind than Nabokov. Here's a bit I like from The Road (which I don't think is a great book all things considered, but it does have some beautiful writing):

When it was light enough to use the binoculars he glassed the valley below. Everything paling away into the murk. The soft ash blowing in loose swirls over the blacktop. He studied what he could see. The segments of road down there among the dead trees. Looking for anything of color. Any movement. Any trace of standing smoke. He lowered the glasses and pulled down the cotton mask from his face and wiped his nose on the back of his wrist and then glassed the country again. Then he just sat there holding the binoculars and watching the ashen daylight congeal over the land. He knew only that the child was his warrant. He said: If he is not the word of God God never spoke.

When he got back the boy was still asleep. He pulled the blue plastic tarp off of him and folded it and carried it out to the grocery cart and packed it and came back with their plates and some cornmeal cakes in a plastic bag and a plastic bottle of syrup. He spread the small tarp they used for a table on the ground and laid everything out and he took the pistol from his belt and laid it on the cloth and then he just sat watching the boy sleep. He'd pulled away his mask in the night and it was buried somewhere in the blankets. He watched the boy and he looked out through the trees toward the road. This was not a safe place. They could be seen from the road now it was day. The boy turned in the blankets. Then he opened his eyes. Hi, Papa, he said.

I'm right here.

I know.

An hour later they were on the road. He pushed the cart and both he and the boy carried knapsacks. In the knapsacks were essential things. In case they had to abandon the cart and make a run for it. Clamped to the handle of the cart was a chrome motorcycle mirror that he used to watch the road behind them. He shifted the pack higher on his shoulders and looked out over the wasted country. The road was empty. Below in the little valley the still gray serpentine of a river. Motionless and precise. Along the shore a burden of dead reeds. Are you okay? he said. The boy nodded. Then they set out along the blacktop in the gunmetal light, shuffling through the ash, each the other's world entire.

Also John Le Carre, and I'll second Terry Pratchett.