Ah the time has come for a post so long that no one will actually read it... HUZZAH!!!
Could Germany have defeated the USSR? I shall give the best case scenario along with all that would have needed to change:
Part 1: The Preperation
1. The Germans would have had to make the decision to attack the USSR right after France and then spend the next year planning the offensive. This would have allowed them to begin fighting right after the Spring thaw. This did not happen since Hitler decided only by (early) 1941 that he wanted war with the Soviets, the Germans spent the rest of the half-year planning a doomed war.
2. The Germans would have had to renounce much of Hitler's policy of "living space", as well as most of the racial doctrine and gone for "liberation" from the get-go, herding up White Russian emigres for puppet governments.
3. The Germans would have had to get themselves mentally ready for war in Russia, that includes logistical preperation (and the mobilization of industry for this), as well as heavy production of general supplies, mainly winter supplies, and more effeicient vehicle construction (tanks

included).
4. The Germans would have had to avoid spending manpower, industry, time, and planning on certain "solutions" final or otherwise... (as roundabout as I can put it).
Part 2: The Actual War
5. The Germans would have had to launch the war in mid spring and kept moving until mid fall, when the mud would become intense, at that pint they would have to stop, dig in, reorganize their supply lines, and make headway into organizing the "free people of Russia (and minors)" into supportive populations.
6. The actual movement would have had to be strictly to:
A) Reach Kiev and take out Eastern Ukraine and move (if possible, note the big if) upto Volga, set up, dig in, wait out the winter.
B) Reach Leningrad and besiege it, hoping that the people "freely join the Germans against the Bolsheviks" also settle in for the winter
C) Reach Moscow and stop, the Germans would have had to dig in and wait out the winter, hoping for political trumoil to erupt
7. Once the winter and thaw end, the Germans would continue pushing East (and if possible through the Caucus), but stopping and digging in periodically to bring the supply lines up to speed, hopefully avoiding
situations in which the Wermacht would outrun its own supply lines.
Part 3: The Peace:
8. The Germans would demand the liberation of several nations (Russian minors as well as Baltic states), the Germans would impose a Brest-Litovsk-esque/Versailles-esque peace on the Soviets, asking for general de-militarization and/or a de-militarized zone adjacent to the German-Soviet Border (so that the Soviets would not attack at some later point), the Germans would have to also oversee all the enforcments of such a treaty.
***
Now the problems with the above are several:
First and foremost it would require that Nazi-Germany not act like Nazi-Germany, which is at best wishful remeniscing.
Second it would require much more manpower than the Germans had at any point, the best hope that the Germans had was that occupied provinces would occupy themselves, i.e. through puppet govs, but the historical track record of foreign-imposed puppet govs is very poor.
Third, there would need to be an admission on the part of German leadership that total control/victory over the USSR was impossible and that all the Germans could hope for would be a quick treaty that would eliminate the Soviets as a military threat. This obviously did not conform to Hitler's mentality or the general German mentality of that time.
Finally there is the fact that the Soviets would never except such a treaty, pitting the Germans against at best a very long, pointless, and doomed war of atrition, at worst a quick defeat (i.e. Berlin in '43) and the historical scenario somewhere in the middle.
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Some other points in no particular order:
The Lend-Lease sipments were a great help to the USSR, but that was not what won the war, the Lend-Lease shipments allowed for the war to end faster and economized many lives (which is how I would put that phrase in Russian to convey its total meaning), all these things are
extremely important, but these did not
win the war
solely.
The taking of Moscow would have accomplished relatively nothing, it would have been a nice symbolic gesture, but that would be it, this was not the 16th century when a capital was more than just a big city, the loss of Moscow would not make the people of the USSR any happier to accept the Germans, on the contrary, Stalin would just appeal to the people to "fight hader for the mother land" and the "people" would.
Much of Soviet industry was transferred to the East, as in, past the Caucuses, the part of the country that DarkSoul1984 referred to as a "heap of nothing", perhaps not nearly as interesting as the stepped of Ukraine, the region around Kursk, or the Pripet Marshes,
but full of factories. Lavrenti Beria, to his credit, organized a highly effecient transfer of industry, so effeicient that it got all the the majority of the major industry, safely out of reach of the Germans within a month of the invasion and was running at full scale by winter 1941, though I am not saying that Beria was a nice man, mass murder being just one of his many misdeeds.
The elimination of many officers in the Great Purge is over played as a cause of the USSR's early tactical defeats in the Second World War. The great majority of the officers killed were old generals from the civil war era, the very same that lost the Polish-Bolshevik War of 1920, nothing genius-like at all. Furthermore their "forward-looking-ness" was not required as the USSR managed to build Tanks, Dive Bombers, and Fighters, well enough without them.
Many German generals (most) did blame Hitler for the loss of the war, but their word should not be taken as the absolute truth. Guderian, for example, often did not see the entire Grand Strategic picture, as shown by his naive view that Moscow should have been attacked before Kiev (a long discussion on this can be found in the "Could Spain have saved Germany" thread, pages 3-5, I think), finally it is easy to blame others for one's own mistakes. Not to say that Guderian was not a brilliant tactician and superbe low-level-strategist who wrote one hell of a memoire, but he himself admitted that he was only human.
Letting "kill ratios" speak for themselves is a very close-minded and childish way to look at a war (any war). The object of any war is victory, not one or two miracle operations, one-sided battles, or even amounts butchered, an almost-victory, is a euphemism fo defeat. The Germans could win a battle and save their own lives with the best of them, but they had a very poor understanding of "the long run", "total war", "international politics", "multi-front warfare" and "logistics". Once a Prussian-style army fighting relatively small-scale wars, always a Prussian-style army fighting relatively small scale wars (in over its head, knee deep in snow and excrement).
To sum up, the Germans were great at small battles and "quickies" (wars), but were easily confused by very big numbers.
The Germans always operated their different fronts in relative independence, with no understanding of "Operational Art" in the Soviet sense, as in Operation Bagration, where several fronts worked together creating huge "feighnts" and fake attacks, concealing the main axis of attack, effectively confusing and destroying the Wermacht (in that order). And, before anyone says this, Operation Bgaration was not an over-done operation on an already broken enemy, the Wermacht was still an effective (if injured) fighting machine before the operation, it was the total success of the operation that effectively castrated the Wermacht (also note that the Germans took loses between 4 times as great [by German estimates] and roughly 6.5 times as great [by Soviet estimates] as Soviet ones [speaking of kill ratios]).
Let us also remember that the Germans slaughtered civilians rather mericlessly and executed Soviet POWs, something that was not done to the Germans (at least during the war), by the Soviets or Allies, this made Soviet losses very high (21 million by Soviet estimates, which includes civilians, 28 by other ones, 8 million of which were military). So, yes, the Germans did have smaller loses, but that was partially due to the fact that opposed governments showed them the mercy that they refused others.
So, in summation, lets look at this from another perspective, let's let historical outcomes speak for themselves. A certain nation was found broken and defeated after 4 years of heavy fighting (and claiming to superiority), the majority of this nation's ruling party was then tried and found guilty by the world for initiating a war (that brought about its own total downfall), this would have been more pervasive if it was not for certain elements fleeing and being prtotected by certain allied governments. Meanwhile another nation found itself catupulted from "desert of back-woods" status to superpower status and would stay that way for half a century to come.
So who really did better?
EDIT: To von_Manstein111 in particular, if you are annoyed by "Russian rants" (i.e. someone stating the remote possibility that the Germans were not the greatest thing since sliced bread) than do not read them, I (and possibly others) may be annoyed by what I
could call pro-german rants, furthermore, I
could lable you a NSDAP sympathiser, but I won't becuase I know better than that, perhaps this discussion is deeper than "my side is better than your side".