What if Stalin took the initiative and struck west before Germany had a chance to organize Barbarossa?
Well this would assume some forward planning on his part rather than Stalin waking up one morning and going 'ah sod it lets invade Europe'
So I dont think its healthy to simply say as some have suggested that the Red Army was not ready in the Summer of 41 - if the decision had been made then they would have been far more prepared that the utter shambles that they were when the Germans invaded OTL.
Lets say that Stalin believed the warnings of Sorge backed up by the British and US intellegence services as early as Aug 1940 rather than putting his fingers in his ears and going "lalalalalalalala I cannot hear you"
An attacker has all sorts of advantages that hide a multitude of sins.
They can choose where they are strong and where they are weak because they know that they are attacking and when and in most cases initially the Enemy would be to busy reacting to take advantage.
In the OTL German Invasion - Stalin had gone to great lengths to stave off any possibility of giving Germany an excuse for war and as such the Red Armys dispositions, supplies and readiness for war was pathetically inadequate in the Summer of 41.
Indeed many Russian formations were hamstrung by their inability to maintain their tanks and supply their artillery beyond one battle due to lack of preparation, lack of transport and a lack of supplies.
We can talk of the German army's advantage of Mission Command and Skill at arms relative to the Red Army etc but if caught unprepared for an attack then initially the German Army is going to be in trouble.
Also Russia had at their disposal 5 million + men underarms and 25,000 tanks.
Far less than half of this was active in the Wetern Russian region and effectively at peacetime readiness
In OTL June 1941 the tanks were scattered about Russian in peacetime garrisons etc and in most case lacked spares and enough transport to support them in battle.
Had Stalin decided to get the first blow in 'May/June' 1941 then initially I would expect things to go the Red Army's way as they would be far more organised and prepared for combat than June 22 1941 OTL found them.
Red Army Armoured units would have been properly re-organised with enough supplies and transport, increased numbers of troops would have been brought into the area and also given improved suppies etc
Much has been said regarding the Russian tanks mostly made up of T-26, BT-5 and BT-7 tanks etc that made up the bulk of those tanks not being as good as the German tanks (lacking radios etc) - it has to be said that the 3500 odd German tanks most of them where light tanks as well such as the PZ II (782), PzKpfw 35(t) (160) and PzKpfw 38(t) (660)
Only 976 were PZ III (with 37mm or 50mm guns) and 439 were PZ IV
http://www.achtungpanzer.com/battles.htm#barb
However they are significantly out numbered
Now all that being said - I have no doubt that the German Army would not shatter under such an assault - but it would be subjected to all of the initial disadvantages that an Army suffers when attacked - particulalrly if the Red Army manages to pull of its desired Deep Battle effects and I would expect that by the time it rallys, concentrates it forces and begins to impose its advantages of training and equipment on the Red Army its going to have suffered significant casaulties.
Also this fighting would have taken place in Poland and not 'Russia' and I suspect that by the time the German Army turns things around - its too late for any chance of a German Victory in the East.
Certainly I do not see Moscow, Lenningrad or Stalingrad being threatened at any point in this example with subsequant major ops taking place much further to the West.