American revolution I guess has a shot. The problem is that unlike the 13 colonies the population Canada (at that time really still New France) dont really want to join the revolt becuase they are afraid that they will lose the rights to language (French) and religion (Catholicism) that the British had given them. Taking Canada in this war would be a conquest not a revolution. The French also want the UK to maintain control of Canada during the revolutionary war (as a means to balence the US in north America) and the Revolutionaries are pretty dependent on French support, in particular needing the French Navy to challenge the Royal Navy (See Yorktown for a great example of a direct effect of the French Navy on the American Revolution). However I doubt the French would actually withdraw support just on the basis of the US taking Canada, their primary goal was to use the Revolution as a means to inflict a humiliating defeat on the British and with the US having enough troops on the ground and the backing of a European Great Power I would argue the Revolution is perhaps the best early date for the US to conquer all or most of Canada.
In the War of 1812, it wouldn't be hard to get the most of Canada up to Quebec City conquered by US troops. The American army and militia far outnumber the British regulars and Canadian militia. To be blunt the US forces SHOULD have conquered much of Canada, they were just kneecapped by incopentent leadership (eg. Hull at Detroit) and a general unwillingness of the militia to fight on foreign territory as well as an irrational fear of Britian's Native Allies (See Queenston Heights and Detroit). The real challenge in the US conquering Canada in 1812 is being able to hold it after Napolean is defeated. Now they face a Great Britain that has defeated all its enemies in Europe and is in a dominant position. With Napolean gone, they are able to deploy the large, well equipped and battle hardened Peninsular Army (perhaps under Wellington) to North America and with the Royal Navy freed up by no longer having to blockade France and fight Napoleon's forces at sea, they can now impose a really crippling blockade on the US. In this situation the US faces a fully mobilized Great Britain and has no possibility of having another European power intervene on the US's behalf like in the Revolutionary war. Considering they managed to hold their own at this point against a newly minted Superpower, I think the Americans did very well here. Maybe a few more Canadian victories would have gotten a more favorable US/Canada western border in negotiations after the war (54-40 or Fight!), but I dont see the US rally having much of a shot at outright conquering the already populated areas of Canada here, they just don't have the strength to do so
The Civil War seems like the worst possible time for the US to invade Canada, do they really want to bring Britain (and France as Napolean III told London that he was willing to declare war on the US if he had British support- mainly because the US was opposed to his intervention in Mexico) in to the war against them while the country is tearing itself apart. The British wouldn't even have to put boots on the ground to seriously hinder Union war efforts, as they would have a big effect just by making the Union pull forces from fighting the confederacy to guard the Canadian border. Also the RN would quickly break the Union blockade of the Confederacy and establish their own blockade of the Union. This is probably the single worst time in the 19th century for the Union to make a play to conquer Canada.
The Venezuela crisis in 1895 was probably the last time there was really any chance of a war between the UK and the US and if war did break out the US (now the world's biggest industrial power) probably would be able to conquer Canada pretty quickly and the UK wouldn't really be able to do anything to prevent that. Although the USN is still dwarfed by the RN and the US would once again face a British blockade, so I dunno how much of Canada the US would eventually manage to keep at the eventual peace negotiations.
In the 20th Century the US can conquer Canada pretty easily and by the 1920s Britain cant really do anything about it (See the US War Plan Red). A WW1 exhausted Britain wouldn't even be able to control the seas as the US would eventually be able to just out build them. Of course by the 20th century US-UK relations are pretty good and there really isn't much of an opportunity to get a casus beli to invade Canada, although the US certainly had the military power to do so.
So the issue is that in general, as US military power grows throughout the 19th century and the first half of the 20th century, the political and public will to up and invade Canada diminishes as relations with the UK improve (and ever increasing economic ties to Canada itself make an invasion even more unlikely- it is to this day the largest bilateral trading relationship between any two countries on Earth). By the late 1930's Roosevelt is making a public declaration that any foreign power (aimed at Germany) attempting to invade Canada would face a declaration of war from the US (See Roosevelt's Kingston Address).
In my opinion the invasion of Canada has its best opportunities for success in the American revolution or perhaps during the Venezuela Crisis (if there is enough political will for it)
Therefore it was was certainly NOT an American Sealion, which had exactly 0% chance of success, as the Germans didn't even have real landing crafts, forget about air or sea control. Indeed the river barges from the Rhine they planned to use to transport the troops would be sunk just by the wake of a passing ship, meaning the RN could sink the lot without even have to fire a shot. Even if they miraculusly got the barges to the shore of England (mind you with tanks or heavy artillery as the barges couldn't handle that in the Channel, so the Germans would pretty much have to rely on infantry to conquer Southern England) there was no way to resupply the invasion force, as once the RN Home Fleet reached the channel, the Germans simply didn't have the surface ships or planes to stop the British from sinking everything with a Swastika. Even Hitler, who was a terrible strategist, saw what a dismal failure this plan would be and called it off. The British and German militaries actually wargamed out Sea Lion in the 70's. Their conclusion was that it was a "Dunkirk in reverse".
