The low countries is an obvious one as the only serious invasion route to/from Northern France hence why they have featured so prominently in British military history. Within the land to the north of the Ardennes the area around Gembloux, known as the Gembloux Gap, offers a stretch of relatively flat, uninterrupted land between the south-north rivers of Holland and Belgium and the river Meuse which runs east-west before turning south at Namur.
Around this area you find the battlefields of Ramillies, Waterloo and the focus of the German Army Group B's advance in 1940.
Assuming you don't just mean "Europe" as in the "Europe = Western Europe" sense of the word, there are two big routes in the Caucasus that were used for this precise reason: The Darial Pass and the Derbent Pass. Together, these two passes formed the main/only two routes through the Caucasus.
The Darial Pass is the gorge at the foot of Mount Kazbek, and it is basically hemmed in by towering rock walls and run partially through with the Terek River. However, it is flat enough and wide enough inside to allow the passage of armies through it. The Georgian Chronicles called it names like Darialani or Dargani. Most of the middle east called it the Gate of the Alans, while other historians like Strabo gave it names like Porta Caucasica or Porta Sarmatica. It is the only militarily-useable route across the Caucasus, and has been fortified for most of the region's recorded history until today. It also houses the Georgian Military Road, which links Tbilisi and Vladikavkaz. There do exist other passes today, but they were mostly constructed from the late 19th century or later, and often are closed during the winter months due to the threat of avalanches or heavy snowfall, which leaves the Darial as the only consistent passageway through the mountains that separated the Russian Steppes from Anatolia.
The Derbent Pass is the gap between the Caucasus and the Caspian Sea located near the strategically-important city of Derbent (Located in Dagestan near the Russian/Azerbaijani/Georgian borders). However, this pass is more often known by its much more popular name, The Caspian Gates. The area has been used to regulate Eurasian/Asian traffic for millennia. The Persians or their client states have intermittently controlled it for most its recorded history. Even the name "Derbent" is a bastardization of the Persian word for "Gateway." The Persian Sassanians also built giant fortifications there (The Great Wall of Gorgan), which are over 100 miles long and contain over 30 fortresses. This pass is also commonly identified as the Gates of Alexander in numerous stories throughout time, especially in reference to Gog and Magog.
While there are undoubtedly secret little routes and footpaths and shepherds trails that cut through the Caucasus, and a number of recent (150 years or younger) routes that have been made: Historically, when one needed to cross the Caucasus and couldn't just go around the mountains in a ship, they used one of these two ways.
Battling the Elements: Weather and Terrain in the Conduct of War (by Harold A. Winters and Gerald A. Galloway, Jr.) has one chapter called "Alpine Fighting and the Road to Moscow". They point out that Napoleon's march to Moscow goes they way it does because it follows a ridge between north-flowing rivers and south-flowing rivers, attempting to avoid river crossings and avoid the marshlands on either side. They then write that the main German drive to Moscow had many of the same problems with the terrain (for logistical support if not for the tanks), and the Germans's ended up making use of much the same path.
The route between the great rivers of European Russia and Eastern Europe from Berlin through Warsaw, Minsk, and Smolensk to Moscow is one such. Historically, it was most notably employed by Charles XII, Napoleon, and German forces during Operation Barbarossa. It's been used to this extent (despite, by 1941, a track record which indicates that defeat lies at the end of that path) because of ancient glacier activity and its impact on geography. This land bridge or river gate is on slightly higher ground than the rivers to either north (Nieman, Daugava) or south (Dnepr), and was carved out by the Scandinavian Ice Sheet over 10,000 years ago. Because of the variations in altitude of the underlying bedrock surface, the path isn't straight but rather sinuous between the various rivers (most notably indicated by the raised ground going around the headwaters of the south-flowing Berezina River)--which forces either significant detours or having to cross the Berezina (which Napoleon was forced to do, with enormous casualties). Also notable are the Pripet Marshes south of the river gate, and the lake districts (such as in Latgale, eastern Latvia) to the north, also a product of the ancient glacier. Napoleon never left the glacier-shaped landscape, nor did much of the Wehrmacht in the east. Only Charles XII left it, and then he managed to do so only by marching south deep into Ukraine.
Thus, even though there aren't any mountains or other really dramatic obstacles, geography still dictated an optimal invasion route.
Source: Battling the Elements: Weather and Terrain in the Conduct of War, by Harold A. Winters with Gerald E. Galloway Jr., William J. Reynolds, & David W. Rhyne.
/edit: Ahh damn, I just noticed that this answer was already given!
Present-day Edirne in Turkey comes to mind. John Keegan calls it 'the most contested spot on the globe'. It has been the site of 16 major battles, because it controls the approaches from Asia Minor into and vice versa through the Balkans or along the black sea from what today is Russia. It's at the confluence of a few rivers and valleys that present the ideal route for armies advancing towards or from Istanbul in various directions.
I am not sure if we know the exact route but both Hannibal's route east across the Alps and Caesar's route going in the opposite town are likel to have share some parts. The town of Gap in France I believe was used by both leaders. Hannibal's route is less understood, but my opinion it most likely went over the Alps at the Clapier pass and others suggest the Col du Traversette.
