Grant will ranked among the great captains of history and definitely one of our greatest generals in the history of the Untied States. There is no question on that accord.  He was one of the first generals of the Civil War to realized that destruction of the enemy army and not captures of geographical locations were the key to victory. His victories were notable because he captured or destroy Confederate armies in its entirety. Only General George Thomas were able to repeat Grant's type of victories at Nashville in December 1854. That should say a lot about how rare it is to destroy enemy's functional field army.
And Grant did it 3 times!!

Of course, Grant made his errors. He totally mishandled Cold Harbor and the Battle of the Crater. Lee was able to fight to s stalemate at Wilderness and Spotsylvania. But Grant was willing to trade losses and was a master of attrition warfare.  

But he won the war and he was generous in peace. That put him up there among our best.