Personally, I've been undecided between Izzet, Orzhov and Azorius, mostly. All of them have some really powerful cards and would seem to fit my preference of playing a more control/tempo deck. However, one of my other favourite playstyles is playing a war of attrition, which led me to look more into the other guilds.
Golgari is what's stood out to me now, and what I'll probably be playing this weekend. It has such good synergy with the options it has bar Orzhov, really. The interactions it has with Dimir, Simic and Gruul are incredibly powerful and shouldn't be overlooked. I find it to especially be the one RTR guild that pairs well with Dimir. Just about ever good Dimir mill card (So the ones which have mill attached to them as opposed to mill being the only thing they do) targets any play, which plays a good role with Scavenge. It's also an interesting combination with the possibility of getting Crypt Incursion in a sealed pool, giving the deck huge life longevity.
Golgari also has the most solid playables in all its guild packs, I think, given the information from this: https://docs.google.com/spreadsheet/lv?key=0Apln2HM4aSGbdDJ6MFk5cTBjb09sXzFWdndhTkx2V1E&toomany=true#gid=0
Also, Golgari's synergies with Simic's +1/+1 counter interactions is hard to miss. Along with Gruul's efficiently costed creatures, and Gruul War Chant, which I've taken a much better liking to than I did previously, at least in limited. It gives you such great value in an attrition war where you're the one playing the biggest creatures; Golgari's speciality, really.
Golgari's interaction with Orzhov is pretty much nonexistent. All it gets is good white creatures, and a lot of removal. So it's not a bad pairing by any means, it's easily just as good as the others, just less exciting. Overall, I'm hoping on getting Dimir as a secret ally, for the fun interactions and the alternate way to winning coupled with evasion.
Being Green/Black also gives Golgari the best half of just about every split card it has a colour in, with the only exception really being Flesh // Blood. This essentially gives it a larger number of "playables" but that's not a compelling argument given how deep sealed pools will be with easy access to playing 3+ colours.
So yeah, I may have written a bit too much, and made it seem more like I'm choosing Golgari for the competitive aspect as opposed to finding it fun, but I honestly wouldn't pick it if I didn't find the playstyle fun. But, as the title says, what are you lot picking and why?