Tom Kean Jr., Runyan, or Garrett.
Garrett could never win a statewide election. He might run for his old seat again, but he annoyed a lot of NJ Republicans and the national GOP this year, so I doubt they would want that. His blanket homophobia is close to toxic in a good portion of NJ (and a good portion of the 5th district).
Runyan could probably do well, but I haven't read anything of his future plans. I've met Tom Kean Jr. and he seems okay but pretty overrated. A lot of the old hype about him was because of his father, who left office long before many young voters now were born.
I personally think that Jon Bramnick is a much better politician and would be a much more effective candidate for the GOP in 2017 or 2021.