Realistically, I think that all the color pairings have a shot. It's still far too early to make heads-or-tails out of what new decks will come out of BNG.
My current sense is the following:
1) Mono-Black Devotion just got the pieces it was lacking to benefit one of its worst match-ups, aggro. As a result, there will be no shortage of MBD players out there. That and everyone already dropped the cash to build the damned thing before BNG...
2) Mono-Blue Devotion didn't really get much in this set and, much to their dismay, actually lose ground as MBD scored some rather effective removal against them. Whereas it was a 50/50 match-up before, I really think it's probably a 70/30 split in MBD's favor now... meaning that Mono-Blue is likely going to decline a fair degree. It won't disappear completely, but it's going to be less of a powerhouse than it has been as of late.
3) Azorius Control also got stronger with added card draw (Ephara) and, most importantly, its scry lands. Expect this deck to continue being competitive and it should grow to a larger portion of the meta.
4) Other fringe decks that were teetering on the edge of Tier 1 either just got the bump they needed to be Tier 1 or fell clearly down to Tier 2
- RDW - clearly Tier 2 for the time being
- R/W devotion - likely very low end Tier 1 or Tier 2 (it just isn't consistent enough)
- White Weenie - likely just made the jump to clear Tier 1 (did you pre-order Brimaz at the low-low price of $25 each?)
- R/G devotion/stomp - Xenagos, God of Revels has completely transformed this build and I envision it
moving up to clear Tier 1 standing
- W/G aggro - this was a well suited deck before and now it has better mana... likely a clear Tier 1 deck
- Orzhov humans/mid-range - both appreciate a few cards from BNG, but Drown in Sorrow really hurts
them. They'll likely hang around at the lower end of Tier 1, but will be less common.
- Others? Well, that's certainly a good question...
5) I envision some other build(s) coming out of nowhere to really put up good numbers. Whether it's U/B control/mill, some heroic build, Conley Woods' G/B "Dredge" that he refuses to let go, or something else that comes out of left field... well... this is sort of the point of brewing
So, what's the best route to go? I can see where R/B is well suited against a fair number of match-ups but you really have to choose whether you're going a more mid-range/control direction or aggro... sadly outside of getting access to both Anger of the Gods AND Drown in Sorrow there isn't much sense to go this route. B/W humans aggro was a better aggro build than R/B aggro and it's not like Rakdos scored a major hit with BNG. For the mid-range or control build, you get some added power, but MBD really just does that better. So, outside of a few tools here or there, I don't see Rakdos dominating anytime soon.
R/W is inconsistent in how it plays. As I heard from an opponent at GP Vancouver, you either pulled the nut draw and nothing could stop you or you didn't and you probably lost... not exactly a recipe for continued success.
R/G I think is the safest route to go, but also likely the most followed path. Xenagos really is that good. Things like Savageborn Hydra and Kalonian Hydra can literally end the game the turn they hit the field. MBD only has so many instant speed removal options and the haste from Xenagos really just punishes them over and over. Mono-blue can't deal with threats which are THAT large on a consistent basis, nevermind siding in 3x Mistcutter Hydra and pairing them with Xenagos. You have Anger in the SB for more aggro decks and just waltz through games.
Personally, I like the idea of going full three-color either into Jund or Naya as some of the combinations are just nasty (and you have almost all the temples you need!). Sylvan Caryatid is fun to accelerate out a T3 Desecration Demon or to pair Xenagos with said demon is fun. Toss in Rakdos's Return and you're having a good time (don't forget Bile Blight, Drown in Sorrow, Anger of the Gods, Dreadbore, AND Hero's Downfall). The mana might be a bit sketchy at first, but it works out well enough to be powerful. Naya... go read Aurelia, the Warleader and then re-read Xenagos, God of Revels... yeah. Toss in the fact you have all the temples and shocks, Anger, Chained to the Rocks, Brimaz, Smiter, Fleecemane, etc etc.
I can link you to preliminary builds I've been tinkering with in both those color combos if you like, but realistically I think you have all the pieces you need to be REALLY competitive and possibly Tier 1 with those builds.