Note that I'm not asking here which Guild's Alignment you feel you fit, I'm asking which one you'd want to be in.
Founded by parun Supreme Judge Azor I, a human lawmage who authored a majority of the original Guildpact, the Azorius Senate functions as the government of Ravnica and creates Ravnican laws.
Founded by parun Razia, a militant flaming-sword-wielding archangel capable of using flame and light magic who fervently and firmly believes in harmonious coexistence on Ravnica, even if lives are lost and blood is shed in achieving this, the Boros Legion functions as the righteous and zealous constabulary and standing army of Ravnica, as the League of Wojek and Boros Army, respectively, and enforces Ravnican laws created by the Azorius Senate.
a guild of secrecy, manipulation and underhanded deals, a shadowy organization operating behind the scenes to twist Ravnica to its own ends. The guild provides espionage, smuggling, burglary, counter-intelligence, assassination and other illegal services for the Ravnican populace, although its patrons are unaware that they are in fact employing the guild, instead thinking they are employing guildless mercenaries or another guild such as the Rakdos. The guild is so secretive that even its own agents often do not know who they truly work for.
Founded by parun Svogthir, a human necromancer-turned-lich, the Golgari Swarm functions as the agricultural sector of Ravnican society. Following his mortal death, using his mastery of magic, Svogthir reanimated his own corpse and commenced body modification, by incorporating body parts from others, such as the chest of the Gruul Clans parun Cisarzim, into it, so as to create a more-perfect, stronger being.
Before and for a short time after the signing of the Guildpact, the Gruul Clans were a wild and noble guild charged with maintaining the wild places on Ravnica. They were supposed to keep civilization in check. Civilization and the other nine guilds, however, overran every wild place on the planet. This changed the Gruul.
Founded by parun Niv-Mizzet, an ancient, brilliant, capricious, and narcissistic dragon, the Izzet League continuously create and destroy and are obsessively driven by a desire for new discoveries. Being the analogue of scientists and engineers on Ravnica and the masters of innovation and spellcraft, the Izzet are also responsible for the design and implementation of most of Ravnica's infrastructure and inventions.
Ravnican guild of business, where the values of white and black meet, where the dead exist solely to rule. Nearly every business in Ravnica ties back to the Orzhov in some devious way or another. Whilst they bear the facade of a religious group and may well have been a true faith at the signing of the Guildpact, they now worship only profit and power.
Cult of Rakdos
Founded by parun Rakdos, an ancient demon, the Cult of Rakdos attend to menial labor and the services industry, which includes some less-than-savory offerings, such as assassination, as well as more agreeable ones, such as the catering (i.e., food service) and entertainment industries.
Members of the demon-worshipping guild are referred to as cultists.
Founded by parun Mat'Selesnya, a female humanoid-shaped elemental formed via the fusion of dryads, the Selesnya Conclave is a decentralized collective variably described as a "selfless, nurturing, spiritual group" or a "brainwashing nature cult", and involves itself in the establishment and maintenance of peace, life, and unity in Ravnica.
The original purpose of the Simic Combine was to preserve the health of all Ravnican life-forms. They were devoted to the study of life, medicines, and biomancy. But just as life can adapt, so did the Combine. Momir Vig, the Simic guildmaster, was dissatisfied with this original purpose and formed a new purpose: to improve upon life-forms of Ravnica.