Pas de Calais
The Pas de Calais was the most heavily fortified spot on the entire Atlantic coast, it was also the shortest [and therefore most logical] point of attack for the Allies so became the place where the Germans fortified the most with several armoured and crack divisions being centred on Calais itself. It would've been an utter death trap for the allied forces and was thus crossed out fairly early on during the planning of Operation Fortitude South [Buildup and planning of Operation Overlord] Assaulting ports directly by sea was crossed out by the end of 1942 after the disasterous [But infinitely beneficial] Dieppe raid.
Belgium
Belgium only has a tiny coastline [Just 30 miles] and was one of the first areas the Atlantic Wall began constuction. There's only two places of note, with Knokke and Oostende, with Dunkirk nearby. Utterly unsuitable and without the protection of the Channel for the RN to place a cordon around, the likelyhood of Norwegian and other places where U-boats were based sneaking into the landing forces to pick off a few ships is quite high.
Norway
Initially favoured by Churchill as it offered hookup with the Soviet Army on their front, Norway is fairly hard to assault due to it's jagged coastline. However, the numerous Fjords would provide fairly pratical multiple places for the Allies to assault. However, once again this was one of the better areas of the Atlantic Wall and was heavily defended, these defences were boosted by their rugged terrain.
It was decided, after a while that Norway would be discounted from the actual Invasion plans for praticality reasons.
However, Operation Fortitude North would see a fictional Army Group created and placed under the command of General Patton. Small numbers of actual troops, mostly communications units [Plus inflatable trucks, tanks and other vehicles] were dotted all over Yorkshire and Scotland showing a massive buildup of the "Fourth Army Group" complete with unit movements at night and corresponding radio chatter.
Hitler, convinced the Allies would seal off the few U-boat places remaining along with the ability to hook up with the Soviets, ordered some 60 divisions from France, Italy and the Low countries and sent them to Norway.
Operation Fortitude North was a complete success, ensuring that large bodies of German troops would remain away from the main theatre of operations until well after a secured beachhead, indeed units didn't start to arrive from Norway until after the Falaise Pocket had been crushed.
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Basically the only pratical place to have launched the D-Day landings was right where they were.