I have an earlier answer on the medieval equivalent of the Fortune 50 (Fortuna L) that might interest you!

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You probably know one way to answer this question for late medieval/Renaissance Europe: the Fugger and Medici dynasties both got their start in the international textile trade, moved into money management/banking when they had the funds to do so, and greatly aided their coffers through monopolizing regional mining.

There's no question that Jakob Fugger, Cosimo de Medici, and their family "businesses" were stupendously wealthy. But the fifteenth-sixteenth century economic world is far removed from our own in some ways, and far removed from earlier medieval centuries too. A "Fortuna L", to medieval people, would consist of a lot more than what we today think of as businesses.

The first major economic force outside the traditional landed nobility were the Christian religious orders. You might have heard "The Templars were the first international bankers"; let that be an introduction into the economic and political power of monastic and mendicant orders as a whole, and more locally, many individual convents (while others, especially women's communities, struggled desperately with actual poverty).

Religious orders were so successful first and foremost because of a factor in the medieval economy we don't think about today: the economy of salvation. When rich nobles and, eventually, burgher families donated land, money, and goods to monasteries, or paid to establish a perpetual chantry at a cathedral chapel, they were doing so in exchange for a specific transactional service: prayers for the souls of themselves and their family.

Now, we talk about "land"; what does that mean by way of economic growth and success? The obvious first level is agricultural produce. But there were other ways to exploit land. English monasteries, for example, involved themselves in building and operating mills. Mining, or rather the sale of mining rights, could help a house prosper. And especially from the early 14th century on, cash rent from tenants was the direct, desirable way to turn land into value.

The system was not an endless spiral upwards. We often talk about the Black Death as adding fuel to the fire of a late medieval fear of/fascination with death, catalyzing even further a desperation for prayers and Masses to speed one's and one's family's souls through purgatory to heaven. Well, those perpetual chantries I mentioned above--donations to a church or convent to say Masses in perpetuity for someone's soul? They were often funded by a donation of a tract of land. And if that particular plot lost all or most of its tenants to one of the waves of pestilence, no more chantry.

What gives this discussion of monasteries as economic actors a particularly medieval dimension is the relationship between individual communities and the international order. The actual administration of orders varied, of course, and some houses were famously independent of major orders altogether. The Frauenstifte of imperial Germany are one of the most famous examples there. But even among a fabulously wealthy order like the Franciscans, individual communities--especially women's houses--could fall into abject poverty, barely able to support themselves; meanwhile, satirical authors from England to Germany tear apart too-wealthy friars for their love of money over God.

The complexity of defining "corporation" in the medieval economy (as well as defining all the elements in the "medieval economy," as noted earlier in discussions of assembly-line production of paradise-souls out of purgatorial ones) also comes into play when we consider something more akin to businesses: the crafts or trades. Individual workshops, merchant houses, and proprieters are certainly and necessarily one level of economic actor. However, in terms of economic contribution to and political power in late medieval cities, we have to think of "corporation" as indicating the collective trade--that is, the craft guild. Medieval guilds were not the premodern equivalent of trade unions. They served as quality control over products and services produced in a given city, in part to maintain the local craft's reputation in regional and international trade. Guilds were also primary players in the urban economies of salvation and public life, sponsoring religious events, feasts, theatre, what have you--often competing with each other.

Georg Christ's Trading Conflicts: Venetian Merchants and Mamluk Officials in Late Medieval Alexandria shows the difficulty of untangling individual from civic "corporation" when considering medieval economic power. Christ studies the career of Biagio Dolfin, Venetian consul to Alexandria who in official capacity traded in gemstones (which he was allowed to do As Consul), and in unofficial capacity of course also traded in wool textiles (which he had to do as, well, himself).

From my notes, here's a great example of balancing individual profit motives with protecting the overall Venetian merchant corps: a group of merchants complained to Venice/Dolfin that the Mamluks, controlling Egypt, were levying an extra merchandise tax which was most unfair. Of course Dolfin bucked up and negotiated with the Mamluks to reduce it--while spilling to Venice back home that individual merchants (certainly not him) were smuggling merchandise and money sewn into clothing and hidden in other ways, so it had become necessary to levy the tax at a different point in the sales chain. Christ had to make sure he was greasing the gears (which 13th-14th century western Europe was finally beginning to understand again!) with the sultan in order to be able to win Alexandrian court cases and aid Venetian merchants overall; he also had to advocate for Venetian needs in order to preserve his status as consul.

Two patterns that have emerged in examples so far of economic powerhouses will come to the fore when we differentiate craft guilds. Here we have a nice guide to hierarchy, since London was generous enough to future historians to establish a hierarchy of its Worshipful Companies in the early 16th century. Merchants are on top, which probably doesn't come as much of a surprise. It was much more beneficial to be a trader of other people's labor than to deal directly with laborers. Furthermore, it was obviously beneficial to be involved in an industry that could take one outside the bounds (physical and fiscal) of an individual city. While there were some luxury specialty items that garnered an international clientele for the best of the best artisans--/u/WARitter has some amazing posts on the international reputations of individual armourers, especially from my beloved Nuremberg and Augsburg--throughout the later Middle Ages, the great European trade icon comes back to textiles. It's not an accident, indeed, that the Medici and Fugger banks got their start in textile trading.

The thing about textiles is, sure, lots of places have a local wool or linen production industry. Despite the predominance of northern Europe in the various steps involved to make workaday cheaper cloth (from growing wool up to dyeing), even a city like Florence, Italy, marshalled enough ground-up cheap wool production to organize an entire guild around (the Arte della lana). The cheap wool and fustian was produced locally and sold almost exclusively locally.

But the real reason textiles became such an important international trade item is that everyone wanted better textiles from somewhere else. This isn't even just European. When white traders arrived in West Africa, they didn't actually have anything to trade that the Africans didn't already have. But they wanted different options for products like textiles. And so with Europeans.

Thus Florence's other textile merchant guild enters the picture: the Arte di Calamela. These merchants were not involved in managing the sheep-up production of fabric. Their traffic was in luxury woolens, in producing a finished product. For the most part, they just bought fine woolen fabric from northern Europe--everyone loves the English not-quite-finished good stuff!--and brought it back to Italy to concentrate on making it from fine to finest. Crucially, dyeing the fabric was one of the most lucrative ways to add value. In fact, when you look at ship cargo manifests from the early Atlantic trade, over and over the commodities acquired are various woods used for brighter and brighter dyes! John Munro points out that Florentine luxury wool merchants could double the value of their purchased wool by dyeing it the most desirable (expensive) shade of scarlet.

There is a lot we still don't know about the medieval European economy. Michael McCormick's famous thesis in The Origins of the European Economy, that slavery and slave trading helped power an international European economy through our near-blackout in sources and jumpstarted the "commercial revolution" has recently been cast into doubt by Alice Rio (Slavery After Rome), for example. Horden and Purcell's suggestion that "invisible" local trade up and down the Mediterranean coastline served that purpose has been difficult to investigate because, well, it's invisible in surviving sources. But two things that are definitely apparent are, first, the complicated meanings of "corporation" in economic analysis. And second, he fact that some trades were better than others, banking could make you the real money but first you had to get the money from somewhere, and some corporations in medieval Europe were rich enough to buy islands--and the papacy.