The Counts of Flanders were indeed some of the most powerful and wealthiest lords in all of Europe, which has a lot to do with the geography of the area and the attitude of the counts toward their peasants. To explain why the count was so powerful, I have to explain the economics of peasants and manorial estates.

In most of Europe, most peasants were tenant farmers, which means they rented their land from the lord who held it in the form of a manorial estate. Or they were serfs, which meant they were compelled to work the land by the estate. Comparatively few peasants owned their land outright, because it was advantageous in the short term to rent it out rather than sell it. It's worth noting here that for most lords this was a matter of economic necessity, they were expected to have a retinue of men, an array of horses both as pack animals and warhorses, and were financially responsible for maintaining everything on their land. It was common for knights with only one manor or castle to barely be scraping by, or for them to be in debt to afford the military equipment they were expected to maintain - one good warhorse could easily cost half an estate's entire annual income and knights were expected to bring several of them on campaign. Noblemen in the High Middle Ages were constantly looking for ways to expand their tax base. For example, a southern French knight called Bertran de Born complains in his poetry about merchants and free peasants, but we know from financial records that he held an annual trade fair that accounted for most of his yearly income. He hated doing it, but he had to because otherwise he'd be broke. Most noblemen (who did not share Bertran's contempt for free peasants btw) knew that free peasants were more economically productive, so they would have more money which meant more trade which meant more tax in the long run despite losing rent income in the short term. They knew that broadening their tax base was better than putting the economic ball and chain of rent on their peasants, but they rarely had the surplus income needed to make the leap even if they wanted to.

I mention this because it's not a situation the Count of Flanders was in. The trade ports of the low countries, and especially the textiles traded in them, turned a substantial profit that the count could tax. Textiles were the golden goose of Flanders that lined the pockets of the count due to the customs duties that went directly to him. If you bought some fancy clothes, or drapes, or an expensive rug, or basically any luxury textile in northern Europe, it was probably made in Flanders. Cities in other regions turned a substantial profit too, but they tended to be direct possessions of the crown and lacked the economic advantages of being the hub of a particular industry. One of northern Europe's wealthiest cities - London - paid its tax directly to the king's treasury, and no nobleman saw a penny from all that trade while the Count of Flanders was raking in the cash.

Colonising wild land and establishing new towns was a favoured activity of European nobility to try and create trade and increase their tax bases, but they never had the kind of head start that already owning several such towns could provide. Flanders had mercantile hubs like Bruges and Ypres, whereas the best most lords could hope for was a town like Exeter on the south coast of England, which had three market days a week as well as a port, but it wasn't a centre of industry like Ypres was with the textile trade. Most noblemen had only one or two unremarkable towns to get them started with their urbanisation schemes.

So with a strong financial head start, the Counts of Flanders could multiply their advantage. The money from the textile trade funded some of the most aggressive land reclamation schemes in all of Europe. They had no need for the rents of tenant farmers, so they let peasants buy themselves out and gave them the right to clear land for themselves in large strips, until they ran into the strip of another peasant's farm (which was also done by the Count of Holland). By 1300, Flanders had a far greater proportion of free peasants compared to its neighbours (except Holland). And because the peasants owned the land outright and had the right to clear more land, there was a race to claim as much as possible as quickly as possible, which the count was all too happy to encourage with the building of dykes and other earthworks to drain the swamplands by the coast and make them suitable for permanent settlement. Once they ran out of land they just made more. The Dutch generally get the credit for being the best at land reclamation, but in the High Middle Ages the Flemish weren't far behind. On this reclaimed land, new port towns including Nieuwpoort, Damme, Biervliet, and Dunkirk were constructed, which could be supported by the food that could be grown on the new fertile land that accompanied them, as well as by importing grain with all that textile money. Existing towns like Antwerp were also promoted, and canals were dug to ensure that land reclamation did not impede trade.

This created a situation where the Count of Flanders could draw on a comparatively large tax base of free and economically productive peasants as well as a network of mercantile cities. The Count of Flanders was personally richer than many kings (certainly richer than the king of France), and put that money to use militarily. Records from King Henry I of England shows that the count was in a position to rent 500 horsemen to the king in the 1100s. 500 was a lot of cavalry for the time. Even very wealthy organisations like the Knights Templar struggled to get more than a few hundred knights or men-at-arms onto the field, so for the Count of Flanders to just rent 500 to his ally is a big deal. This prosperity wasn't confined to cavalry either; whenever a Flemish army appears in historical accounts, they are always armed to the teeth in the form of well trained and expensively equipped militia when other lords were reliant on calling up vassals or raising a levy. They could even go against knights and win. As the Annals of Ghent noted after the Battle of the Golden Spurs:

the flower of knighthood, with horses and chargers of the finest, fell before weavers, fullers and the common folk and foot soldiers of Flanders

And that the king of England was relying on him to provide cavalry for his army shows how important Flanders was militarily and on the world stage - an alliance with Flanders could easily make or break a campaign, though they had an annoying habit of trying to be neutral or play both sides of a conflict. Flanders was effectively independent; it had the money and the military to do what it liked.

But the Flemish weren't the only people doing well. The French monarchy became more powerful, seizing Normandy and Aquitaine for itself in the early 13th century. Flanders was no longer economically or militarily more powerful than the French king. France now had revenue from the coastal ports of northern and western France, as well as the knights from those regions. This became obvious during the Franco-Flemish War of 1297-1305, which saw Flanders absorbed into France.

The Franco-Flemish War also showed a military disadvantage of Flanders' situation: they had very few knights. The Annals of Ghent allege that, at the Battle of the Golden Spurs in 1302, the Flemish army had only 10 knights. What happened to the days when they could rent 500 cavalry to the English like it was no big deal? Well, when they encouraged peasants to be free and heavily promoted urbanisation, they basically killed the manorial estates that provided knights, and the urban militia of the cities were far more suited to fighting as infantry. This wasn't a problem early in the war, since the Flemish infantry militia were able to defeat French knights, but the French knights adapted to Flemish pike tactics, there were a lot of them (over 1000), and suddenly the disadvantage of having so few knights was laid bare. The pivot from cavalry to infantry militia was a mistake as a way to counter masses of French cavalry. By 1300, the rapid expansion of the French king's domains was too much for even Flanders to resist. This was the beginning of the end for Flanders' power, since rivalry between England and France meant a gradual shift in textile production from Flanders to England, as the English were less willing to trade with Flanders now that it answered to the French king. Then the Hundred Years War began. Then the Black Death happened, and affected the heavily urbanised county of Flanders worse than some of its neighbours.

But from around 1100-1300, the Count of Flanders was indeed one of Europe's most powerful noblemen, thanks to a strong economic starting point and the will to exploit it to the fullest extent. But in the end, the growing power of the French king and the economic consequences of that rise were too strong to fight and, while still rather powerful, it didn't enjoy the independence or level of importance it previously held.