Short answer : Barbarossa '42
Longer answer : From the European perspective, at least, the whole war hinges on the Eastern Front. If the Axis had succeeded there (and they very nearly did), nothing else much would have mattered. So, what could have been done differently?
Firstly, let's look briefly at why the historical campaign did not succeed
1) Delayed start date. The planned invasion date was May 15, 1941. Various events (mostly the Balkan Crisis) conspired to delay the invasion for several weeks (although it is worth noting that it is in fact likely that due to terrible weather in May - April '41 the start date may have to have been pushed back anyway), costing the Axis several weeks of relatively prime campaigning weather
2) Loss of the Fliegerkorps. Due to Enigma decrypts, the NZ Division on Crete pretty well rekt the German parachute forces, meaning they weren't available for action in the USSR in their primary role. Whilst we can never be certain that they would have made a difference, the success of other airborne forces throughout the war implies that having access to them would have been a net benefit to the Axis forces, especially given the scale of the theatre. A delayed Barbarossa would have give the Germans time to rebuild a viable force for use in the East
3) The Kiev pocket. Thanks to undue dithering, infighting and glory hunting in the German high command, several prime Panzer divisions spent about 3 weeks sat around doing nothing in Belorussia (or thereabouts) whilst the German High Command decided if they wanted to go for Moscow or nail the Soviet SW Front. In the end, they tried both, destroying the SW Front as a coherent force, taking over 600k prisoners, but failing to get the decisive victory in front of Moscow that they needed. Ultimately the additional, unnecessary delay cost them their best opportunity to take Moscow and score the decisive victory required to put down the USSR
Obviously, this is a pretty simplified analysis, and I'm more or less taking it as read that only an invasion that successfully took Moscow *in the year the invasion was launched* had a reasonable chance of overall success (by defeating the USSR, leading to its' capitulation and/or degeneration into civil war etc)
So, if Barbarossa had been delayed until 1942, what would have been the pros and cons?
Pro :
1) More good weather to fight in. Restoring the start date of May 15th would have allowed the Axis several more weeks of relatively good weather to do whatever the hell they wanted. Clearly a benefit for the Axis
2) Better Axis equipment. The invading German forces were pretty woefully underequipped in 1941. There were Panzer divisions still using the Pz38t as their main battle tank in at least some of their Pz regiments in 1941, and supplies of the PzIII / PzIV were still limited. These formations were fine for shooting up the BT / BA series vehicles the Soviets still had in service in their silly little Mech Corps, but the more modern Soviet tanks outperformed them, at least on the individual level. Whilst the T-34 and KV series tanks were in service with Soviet forces in 1941, their supplies were also limited, and the early versions really weren't very good, at least by comparison with late-war models and the Axis inventory.
Perhaps more crucially though, the German forces may well have been able to increase the number and quality of trucks in their inventory. In the East especially, supply was often the most limiting factor preventing or slowing Axis advances, rather than Soviet resistance. Even having been able to breed more horses and produce more modern horse drawn carts would have significantly reduced Axis supply problems
3) Missing the '41-'42 winter. That winter was brutal, even by Russian standards. 1942 was pretty wet, which may have gone some way to reducing Axis mobility (one of their chief advantages over the 1941 Red Army), but Axis forces suffered far more losses in men and material due to terrible winter weather than to enemy action in winter '41. As a (very) speculative sidebar here, it may also have been the case that the restoration of the Soviet officer corps may have been even more difficult in 1942 than it was in 1941, as it is likely that quite a number of competent but "politically unreliable" officers would have died in the gulags during that especially hard winter
4) Destruction of the Red Air Force. Although at considerable cost, the Luftwaffe essentially destroyed the Red Air Force in under 2 weeks at the start of the operation. This had less effect than it could have however, since Soviet high command was aware of the largely obsolescent nature of its' equipment, and already had plans, and production lines in place, to replace them with newer, more capable aircraft. Had it been those aircraft that were destroyed, it would have been that much harder for the Soviets to bounce back from the losses
5) A second Eastern Front. Things get even more speculative here, but bear with me. If Barbarossa *had* been delayed, what would the Axis have spent the rest of 1941 doing? Clearly, they were unlikely to have just taken the year off. Well, there were still plenty of other places to fight. Rommel (who was OC DAK at the time) was desperate to get proper support for the campaign in North Africa, and the Med in general. Plans existed for the invasion of other Allied bases in the Med, and Rommel was so keen to see them go ahead that he offered to personally lead them, in order to secure his supply lines and allow his troops in NA to actually do the things he wanted, rather than what his supply situation would allow. Had this gone ahead (and there would have been AMPLE resources to do all of it, rather than just shovelling them into the maw of a land battle in Russia), the Allies could quite well have been pushed largely out of the Mediterranian.
Had that happened quickly enough, the DAK and Italians could quite conceivably have defeated the British in NA, and gone on to seize a lot of the Middle East too with the aid of pro-German (or at the very least vehemently anti-British), and in some cases also pro-Nazi local leaders. That could have opened up potential land invasion routes to the Caucasus and Imperial India, which would have vastly complicated Soviet and Allied defensive plans. The British Empire would have been forced to spend resources defending India, and even a quite small force centered on the DAK could have invaded the Caucasus, denying the Soviets troops desperately need to reinforce their western front, and indeed possibly requiring them to divert troops from the main action in the West to guard their oil supplies, without which the USSR would have been crushed in short order.
It's entirely possible that Turkey would have been co-opted into the Axis as part of such an operation too, which would have been another major headache for the Allies and Soviets in it's own right, let alone any strategic or tactical implications for the invasion of the USSR.
Cons :
1) Second Winter War. There is some evidence that the USSR was intending to renew the war against Finland, and with the territorial gains they enforced in Round 1, and the experience gained, they would very likely have won. The Finns fought hard, and smart, but the Soviet war machine was always going to win a rematch, and that might have had all sorts of ramifications. Of course, it might also have made the Axis High Command take the Red Army more seriously ("We just need to give the whole rotten structure one hard kick, etc..."), which might have been good for the Axis too, in the long run
2) Soviet military build up. Although the T-34 mod. 40 / early KV series weren't great, they were still better than most contemporary Axis equipment, and there would have been more of them. Of course, the Germans would have bee better equipped, as noted previously, but more T-34s is never going to be a *good* thing for the Wehrmacht. Also, the re-equipment of the Red Air Force would have been at least under way, which might have been harder on the Luftwaffe, even if it hurt the Soviets too. The newer, zampolit-approved officers would have had several more months to get accustomed to command, and would likely have been at least marginally more competent too, which again can't be good for the invasion forces
3) Weather. As noted previously, 1942 had a much milder winter, but a very wet summer. The mud was often more of a barrier to Axis advance than Soviet resistance, so maybe it would have been General Mud, rather than General Winter who saved the USSR
4) Other. A year is a long time in war. A lot of things could happen, and they wouldn't all have gone the way the Axis wanted, to be sure. As a big maybe, what would the US have done post Pearl Harbour if the Axis and USSR hadn't been at war? Would it have made it easier or harder for FDR to cement "Germany first" as US policy, or have made no difference? We can't be sure, either way
Would Stalin, fresh from success in Finland, have launched his own pre-emptive strike into the Reich? The thought would surely have at least occurred to him. Again, hard to say, and even if he had, would that just have made the Axis task easier, allowing them to defeat the Red Army in the field before setting one foot on Soviet soil? Or would the Soviet advance have been able to blast a way through Poland and Germany and take Berlin in 1941 or '42?
Edit: Wow, what a wall of text. Sorry about that, but it's a big question
