Specifically, I would like to know in what context did RE Lee opine about McClellan? "Little Mac" certainly showed greatness in organizing, training, equipping, and inspiring an army. Yet, he failed to commit his army in battle for (my opinion) fear of casualties, which might generate bad feelings among a future constituency in his and other states. And so, Casanova Lee may have omitted an addition comment from RE Lee that would have qualified his statement.
It might be worth thinking about the campaigns in which McClellan and Lee faced off against one another.
The first is the Seven Days. This is an environment when McClellan is in the process of breaking into Richmond - day one of the Seven Days is actually McClellan advancing his lines at Oak Grove, preparatory to establishing his siege guns and blasting through the Richmond forts.
The meat of the Seven Days is an attack where Lee got to concentrate a massive force in front of Richmond - the largest Confederate army that ever fought on the field - and hit McClellan in a hanging flank, with the expectation of being able to destroy McClellan's army or at least force it into a position where it was no longer threatening Richmond.
Over the course of the Seven Days, McClellan pulls his army back from having his supply line blown away, avoids the entrapment of his army, and establishes supply on the James River. Lee has removed McClellan from the capital, but he hoped for far more and McClellan is still close enough to threaten Richmond (and Lee acts accordingly) - it's not until McClellan is withdrawn from the Peninsula entirely that Lee feels free to operate, and by the time he was asked the question about "which general" he'd have known that the withdrawal was not of McClellan's doing.
The second is the Maryland Campaign. This is an environment where at the start of September Lee has just conducted one of his masterpiece campaigns, smashing Pope's army and practically routing it back to Washington.
Lee's read his Napoleon - he knows that if you smash an army that heavily then even with an intact cadre to base it on it's going to take some time before it's fit for the field again.
Then comes South Mountain, a strategic thunderbolt by the Army of the Potomac (which has been rebuilt at startling speed) which catches Lee off guard and nearly sees the surrender of a quarter of his army. He's telling McLaws to get his men over the Maryland Heights by any means necessary and ordering Jackson to call off the siege of Harpers Ferry, and it's only the precipitate surrender of HF that lets him rescue ten of his forty infantry brigades; meanwhile Lee has to rush to haul his army back together and on the 17th is the bloodiest day of fighting in American history, with McClellan launching almost all the attacks.
The third is the Loudoun Valley campaign. To go into full detail on this one would take too long, but the fundamental basis of it is that Lee realizes about the start of the second week of November (November 8-10) that McClellan has thoroughly humbugged him. Lee is preparing to pull back to Gordonsville and repeatedly ordering Jackson to join him, and even a week later when Burnside starts moving Lee pulls back clear to the North Anna (to give time for Jackson to arrive) before finally realizing that Burnside isn't over the river at Fredericksburg and he advances to contain the Army of the Potomac.
So Lee and McClellan have campaigned against one another three times. The first sees McClellan rescue his army from the largest army anyone in the Confederacy ever deployed, and Lee ends it with the knowledge that - while he did force McClellan back - he didn't force McClellan back very far, and McClellan is still able to threaten Richmond.
The second sees McClellan outmanoeuvre Lee, which wasn't exactly a common occurrence, and deliver serious damage to his army. He didn't "seal the deal", but then again nobody did until Lee's army was a husk of its former self; Lee has to retreat back into Virginia.
The third sees McClellan outmanoeuvre Lee
again, and the danger posed by the campaign is only arrested by McClellan's firing. Even then when Burnside takes over and starts moving a week later the Army of the Potomac's position (placed there by McClellan) is threatening enough that Lee thinks he has to give up the line of the Rappahanock.
Compare this with the other generals Lee faced - three of them (Burnside, Hooker, Meade) were generals where Lee was able to successfully defend the line of the Rappahanock, a fourth (Grant) was someone who was able to keep outflanking Lee's army but didn't actually manage to outmanoeuvre him and catch him at a disadvantage, and the final one on the list (Pope) got routed.
Finally, consider the issue of sieges. Lee was at Richmond while McClellan was on the Peninsula, and what he saw was that McClellan faced five strongly defensible lines on the route from Fort Monroe. McClellan outflanked one (the Big Bethel line), rendered the second (Yorktown) untenable by regular approaches, outflanked the third (Williamsburg), crossed the fourth (the Chickahominy) and after some awful rain was ready to overcome the fifth (Richmond Defences) about a hundred days after he started the campaign. This is why Lee said that McClellan would "make it a battle of posts" to take Richmond, moving up his guns from strong position to strong position slowly but inexorably.
The other general who Lee faced in a siege situation was Grant, and Grant never really tried regular approaches in the first place.
So yes, I think that Lee would have reason to say that McClellan was the best general he faced, because:
- McClellan had the ability to outmanoeuvre Lee, and did this more than once even if Lee could recover from the bad situations.
- McClellan could recover from a bad situation as well, and recognize when he should fall back rather than staying to fight and getting destroyed.
- McClellan did launch heavy attacks against Lee (Antietam should be proof enough of that) and Lee suffered from them.
- McClellan had skills (regular approaches) that other Union commanders did not prove to have.
- McClellan's army was smaller relative to Lee's army than other Union commanders, particularly Grant; Lee could say Grant won through superior numbers, but the same explanation doesn't wash nearly so well with McClellan's army.
That doesn't mean that given the historical events he
had to have said that McClellan was the best. I could understand if Lee's answer had actually been Meade, or Grant, for example. But what it means is that Lee's statement of "McClellan" is not
unbelievable, and we shouldn't reject it.
(It's sort of similar to why the Duke of Wellington claimed Assaye as the battle he was the proudest of; we could understand if he'd said Salamanca, or Waterloo, or Vitoria, but Assaye is not
unbelievable.)