I don't think the two comments are wholly contradictory, Vonadler's comment (the first one) is more correct, where as the second comment sorta of veers off into a what if scenario. The only place where they fundamentally disagree is with attacking Russia. Vonadler seems to state that attacking Russia was motivated mainly by strategic factors (convincing Britain to surrender) as well as the ideological factors. Where as the second comment, the one from musschrott, states that invading Russia was "the worst strategic decision in history" (cf. Bonaparte, N.)." In this regard, Vonadler is overwhelmingly correct. See my answer here as to why the Germans invaded the Soviet Union.

Its easy to look back with hindsight and say that invading the Soviet Union was an awful decision, but realistically the German leadership's hands were tied in this regard. Any hope of taking out Britain either via U-Boat, or a protracted air campaign was basically zero, an amphibious invasion was in the realm of fantasy. In the Nazi worldview the only way to isolate Britain was via a campaign to quickly knock out the Soviet Union. And they were onto something, Britain certainly was feeling alone and without aid from either the Soviets or the Americans could do very little to physically damage Hitler's fortress Europe. So a campaign to take out one of Britain's obvious allies wasn't the worst idea ever.

Now was Operation Barbarossa unwinnable? Its a question worth considering. Certainly if Germany did have a chance at winning, it required lighting speed and them to be able to follow their timetable exactly. Unfortunately that's exactly what didn't happen. Operation Barbarossa plagued by unclear goals (endless arguing in the high command over whether to take Moscow, or split the main force and take Leningrad and Ukraine), underestimation of Soviet capabilities, logistical and reinforcement issues, and finally the problem of stiffening Soviet resistance that really hadn't been planned for (the Germans expected to deal with the Soviet army early on in the campaign and then proceed to leisurely stroll into the rest of the Soviet Union).

So an invasion of the Soviet Union wasn't doomed from the start, and in fact wasn't even necessarily a bad idea, but the way the Germans went about executing the invasion and the way the Soviets responded to it, ensured that Operation Barbarossa was doomed to failure.

I would consider Hitler's declaration of war on the U.S. at least belongs in the conversation about biggest blunders.