Absolute best has already been covered so I'll go with "realistically possible best" - Trump runs a good campaign with no major foul-ups and picks a decent VP (for giggles, lets say he convinces Nikki Haley to take the VP nomination and she works out well), against a troubled Democratic campaign, where the candidate makes several large public gaffes and can never really unite the party, and the VP choice of Julian Castro fails to not-quite Sarah Palin levels, coupled with a major national disaster (economic, military, or natural) that President Obama is blamed for.
Dem: 233
Rep: 305