Best case for Democrats; Someone competent other than Hillary/Biden runs and wins a tight race (nominating Hillary or Biden is setting the party up for disaster in 2016, or worse 2020).
Best Case for Republicans; Cruz runs and wins the nomination, then, aided by low popularity for President Obama and poor campaigning from Hillary/Biden, wins the General Election easily. (Cruz is probably best case because he is ardently conservative, and would be competitive or better in November-Walker might fit this too, but he might not be conservative enough for most activists)
Do you know anything about politics, at all?