Very honestlly? i think the Best candidate would just be new evolution of the ape familly into a sentient being; after all, we already know its possible for them to do so.
other then the apes, well, bioligiclly, the Octopi and squid familles have thebest biology,as with their large brains, and the ability to manipulate objects with thier tenticles, its obvious they have what it takes in that respect; however, Octopi, the smarter of that branch of species is rtaher solitary, and I myself dont put all that much stock in the squid evolving to be somthign greater then it is,. at least when compared to Octies...
Grey parrtos diserve an interesting mention; while it would take a LOT longer for them to evole into somthign that can dominate the world as a human, in sheer intelleigence, they they are off the wall! Theybecxome as smart as a 5 year old human child, and tests have shown that it isnt the mere trick-based trainign many migth think, but it real intelligence, and each bird has its own little personality; they are also highlly social, another great boon into evolution into a Human type dominante species; the main obsticle is the lack of the real tool for manipulation, yes the feet and beak are there, but in the long run, thier use is limited; however, the fosile record shows that Birds have reevolved arms at least once, and so it woudl nto surprise me that, if given the right enviroment, they could do it agian, and in doing so be on track to dominanace