I think Renault will take it. They are currently 5 points ahead of Toyota, and from a developmental point of view the Renault seems to have come on strongly over the past few races. Plus as has been pointed out they have Alonso who is probably the most complete driver in F1, and he seems to be going from strength to strength again, the side is being let down somewhat by Piquet but I feel that Alonso has things under control.
The Toyota has been a solid car all season long, and both Trulli and Glock are likely points scorers still, but neither of them are a Alonso.
I think the battle for 6th could be just as intriguing. STR are 3 points ahead of RBR, Vettel certainly seems to be on a roll atm, but it would only take a good result for Webber to close the gap again. Both Bourdais and Coulthard are unlikely to score points unless circumstances help them.
Both teams should be safe from Williams, they did well at Singapore but that was probably due to a combination of circuit characteristics and circumstances, their car hasn't developed as well as the rest of the midfield.