I'm a big fan of Ulysses Grant. I think he was truly the outstanding general of the U.S. Civil War and a good case could be made that he literally saved his country.
I think he was the one general who really understood the changed nature of warfare during the Civil War, especially that battles had lost their ability to decide campaigns in one fell swoop. Instead he treated battles as the incidental consequences of his maneuvers, which had higher strategic goals. For a long time I don't think he got his due, especially in comparison to Robert E. lee, who during his lifetime and after was beloved and revered. Grant had the misfortune to live long enough to have a second, rather less successful career as president -- although here, too, I detect a re-appraisal under way that is elevating his reputation a bit.
And Grant seems to have been a thoroughly decent man with personally admirable qualities.
But, like all men, he did have his flaws. And I think his most singular flaw was being an abysmal judge of men. This most famously got him into trouble as president, but it also marred his generalship as well. Grant seemed to have difficulty separating his personal affections from his professional judgments.
He was a big fan and great friend to Sherman, for example. While I don't think Sherman was a bad general, by any means, I think the evidence for him being a great general is slim. While he was able to capture Atlanta, for example, he did have both a substantial numerical advantage as well as a qualitative edge. His truly noteworthy campaign was his March to the Sea and he deserves full credit for that, but his opponents were unable to mount a credible defense against it.
On the other hand, Grant seemed to have a dislike for George Thomas that's hard to fathom. On the one hand, Thomas was slow to act, but that hardly set him apart from the vast majority of Federal commanders. Indeed, one of the undoubted talents of Grant, Sherman and Sheridan was their relative alacrity. But those three aside, most of the top generals on both sides were pretty slow. (Speed was the secret to Stonewall Jackson's success and reputation as well, so it wasn't just a Union thing). And Thomas was clearly an effective and successful general, yet reading Grant's memoirs one definitely sees that Grant was no fan of the man and came close to relieving him of command more than once. Meanwhile dolts such as Butler, Burnside and Sigel were allowed by Grant to stay in command and were given important tasks to fulfill.
So yes, Grant was a great general, but he was also a flawed one.