Costello would probably be the strongest candidate, followed by Dent.
Costello has never faced a tough race before. He has not won in a 2006 or a 2008 in a 57% Obama district.
I am not saying he is a weak candidate, he probably should be on the list. But I wouldn't put him as number 1.
Costello's district is just like R+2 or something, but he's probably a pretty good fit for it. Anyway, Dent is not a strong candidate because the pro-life vote is very influential in PA, and for every "socially liberal, fiscally conservative" vote he picks up in Lower Merion or Radnor, he will lose two votes to his nominally pro-life Democratic opponent.