The Mage's Guild is the most noble, obviously. They're dedicated to making magicka more accesible to the everyday man, which makes everyone's life easier.
Sorry, but I have to call BS on this. The Mages Guild in NO WAY OR FASHION makes accessing spells any easier. If anything, they have been continuing to make it harder, and are a bureaucracy than anything. I don't even need to look at the previous games, OB gives me all the ammo I need. They force you to travel to every single city in Cyrodiil and get recommendations from every leader. To the common man, that's a hard task to accomplish, and only the most ambitious would go through such trials. Not to mention they removed teleportation, so it's not like some novice or associate could pay some guild guide to warp them to another hall. No, they have to walk through hostile country. And if that was not bad enough, each city really only excels in one school of magic, further making it even more difficult to become a well rounded wizard. And if that was not bad enough, you can't access any custom spells or enchanting devices until you get all your recommendations and are allowed into the university. And that's just OB alone. I'm not even going to touch DF or MW.
Granted, not everyone adheres to the guild's founding ideals of nonelitism, but the guild itself exists as a true force for good in Tamriel.
No one in the Mages Guild, except the Ald'rhun chapter who offers to teach reading and writing to common citizens, adheres to the guild's founding ideals. It was originally made to allow those who can pay to come in and learn to be a mage. Hell, the founder left the guild and was thoroughly disgusted with the guild since it pretty much became almost as elitist as the damn Psijiics. The guild is a bureaucratic blob that's becoming more and more restrictive. Not to mention there are plenty of MG members who are not so noble, and have quite the chunk on the shoulder.
The Fighter's guild is just a neutral mercenary force. They adhere to the law, but only because they have to while working in Imperial jurisdiction.
har! The Vivec and Balmora chapters easily took jobs from criminal organizations to act as enforcers and assassins.