In the most recent block, my favorite guild is and will always be Dimir. I like the flavor of it, I like the mechanics. Unfortunately, Dimir is also difficult. If I'm playing to win, like say I wanted to join a tournament, I'd go with the Boros Legion. I like them, too. I have a Boros deck that can take apart just about anything. I struggle against the Delver deck my girlfriend likes to play, but even that is mainly challenging, hardly impossible ( Unlike with my Dimir deck, which more or less consistently loses to that deck ). I have several other decks that I made basically because I got lucky and drew a lot of good cards ( My Orzhov deck has two Blood Barons, two Teysa, and Obzedat, all of which I came upon by chance - I think it was a sign ).
Currently I'm working on building something a bit older, a Naya deck from Alara. Mainly because it's something completely unlike anything I've built so far, and I really like the flavor. Also, they have some insane critters. I mean, the whole concept of Naya is to have creatures power 5 or greater. A bunch of their abilities do things like "Creature with power 5 or greater gains indestructible until end of turn". Now, that's cool. But some of these creatures are just crazy. Like the Meglonoth, which at 6/6 for 3RWG with Vigilance and Trample is already decent. On top of that, it deals damage equal to its power when it blocks, to the creature's controller. Those creatures are insane even by the block's standards!