I assumed/know that Nazi Germany could have easily won the European theatre if Hitler hadn't have interfered as he did. But how about Japan, did they ever have a real chance?
I went into some detail on the topic of Nazi Germany's chances of victory in this post.
An excerpt regarding Adam Tooze's thesis as outlined in The Wages of Destruction (an excellent book on the economic history of the German state over the period):
The central dilemma faced by Hitler as outlined in The Wages of Destruction (Adam Tooze) was that Hitler was unable to defeat the British empire and Americans without access to a secure, independent base of resources, and the only place that could be found was in the USSR (N.b. it was expected that the Americans would be supplying the British with resources and industrial goods, even if they didn't intervene outright). But the same lack of resources that prevented them from outproducing the western allies in the long term also prevented them from being able to achieve their objectives in the east.
The Axis powers did not need to literally invade America, they just needed to have their conquests and territorial objectives legitimized and hostilities to cease. What were those objectives, and how close to fulfilling them did the Germans and Japanese come?
Japan
In 1937, hostilities resumed between the Chinese and Japanese. There had been a conflict resulting in the conquest of Manchuria some years prior (Manchukuo was the puppet state administered by the Japanese here), and in 1937 the fighting started up again. The Japanese spent the next several years attempting to conquer mainland China from both Nationalist and Communist forces.
Beginning in 1938, a series of increasing trade restrictions were imposed on Japan by the United States in response to their aggression. In 1940, this resulted in a scrap metal embargo, which hit the Japanese quite hard. The next step was the much more famous oil embargo in August 1941.
On the Mainland, the conflict with China had descended into a stalemate. A brief expedition into the USSR in 1939 saw the Japanese routed and a non-aggression pact signed with the Soviets (look up: Battles of Khalkhin Gol for more information).
As of mid 1941, the Japanese were quickly becoming resource constrained, and the oil embargo threatened their war efforts further. Thus, the plan was hatched to sieze Oil and other strategic resources from the Dutch East Indies. This would draw them into war with the Western Allies, and the Japanese believed, also with the Americans. To preempt the American response, the attack on Pearl Harbour was planned.
The most ideal outcome for the Japanese, then, would be victory in China before they had reached this point. Since the Japanese military was not particularly capable of overcoming the Chinese in the long term, they did not have much of a chance to decisively win that conflict. A negotiated peace that gave small territorial concessions to the Japanese - but far less than they wanted - might have been possible, however it would be contingent on the Chinese agreeing to this, and once the stalemate set in, this seems implausible.
As to whether the Japanese could have defeated the United States once they entered World War II proper, the answer is a pretty resounding no. The attack on Pearl Harbour was devastating, however the material capabilities of the United States were such that this was a temporary setback for America. So long as they were willing to fight on, the Japanese would be facing daunting odds on account of the incredible ship-building capacity of the US.
Germany
Between 1936 and 1939, Germany reoccupied the Rhineland, annexed Austria and Czechoslovakia, and had the city of Memel transferred to their control. The long term objective of the Nazi state was to create a vast, resource independent empire stretching approximately to the Urals mountains (one of the traditional dividing lines between Europe and Asia). This involves conquering most of the heavily populated areas of the USSR.
In 1939, the Germans found themselves in a war with the Western Allies. This conflict did not necessarily help in acomplishing their long term objective, however the chance to settle scores with the French and dominate Western Europe was siezed upon. For an analysis of Germany's chances to "win the war", I will once again refer you to my previous post on this subject.
I recommend you look at some other posts on the subject via the searchbar, it's a common question. To target a misconception in your post about Hitler however:
Germany could have easily won the European theatre if Hitler hadn't have interfered as he did
Wrong, really wrong. The idea that Hitler was some nutty idiot who stopped his generals from achieving victory is a school of thought largely invented post-WW2 by... you guessed it, Hitler's Generals. The biggest proponents of the theory were various former Nazi German leadership who were very quick to push the blame for the nasty sides of the war (the Holocaust) and their own failures onto the easiest scapegoats post-war, Hitler, the SS and the Nazis. This is where we get those blatant fabrications like the idea that the Wehrmacht was innocent in the Holocaust, or that most Germans did not have any clue what was going on.
In reality, there's not much evidence to suggest Hitler was an exceptionally poor strategist, or that he personally caused the majority of failures, or that the things we know he did cause would have changed the war at all had he not done so. There were certainly foolish things we know he ordered, like leaving so many to die in Stalingrad, but WW2 was lost for the Germans the second they stepped onto Russian soil (and arguably, the second they stepped onto Polish), and no amount of tomfoolery from Hitler or lack thereof was going to change that.
Anyway, here are a few of the questions, already asked, feel free to browse them for more answers:
https://www.reddit.com/r/AskHistorians/comments/3pbuii/what_were_hitlers_chances_on_winning_the_war/
https://www.reddit.com/r/AskHistorians/comments/2ezsaw/why_did_germany_lose_ww2/
I assumed/know that Nazi Germany could have easily won the European theatre if Hitler hadn't have interfered as he did.
Others have already pointed out how Hitler gets a lot of unwarranted blame.
That said: I would argue that Germany potentially came within 3 dud torpedoes of 'winning' the war.
On October 30th 1939 Churchill (then First Lord of the Admiralty) was meeting with other members of the Royal Navy on the battleship HMS Nelson. Group this ship was in was sighted by the U-boat U-56 who managed to end up in nearly a perfect position to fire on the Nelson (800m at a broadside target) and managed to hit her with 3 torpedoes, none of which exploded.
If things had happened differently, and Nelson had gone down quickly resulting in Churchill being killed. You then have to figure out who the next PM would be, and have a very good chance to end up with someone like Lord Halifax who was in favor with peace with Germany following the Fall of France (Halifax seemed to realize that a protracted war would be the end of the British Empire, and valued that empire more than he valued continental Europe).
As far as Germany is concerned, a UK worrying about the British Empire, and a Germany worrying about continental Europe is probably about as "won" as you can get in the West.