Q:

Which factors are most important in getting a high star rating?

Which variables are most important in getting a high star rating on a level in Luigi's Mansion: Dark Moon?

Clear time
Ghosts captured
Health lost
Treasure collected

A:

Your "score" is calculated by this formula:
Score = Ghosts * 120 + Coins - Time (seconds) - Damage * 6

Then, every level has a "score table", holding how much score one needs for how many stars (some levels have even negative scores to get three stars). The exact numbers are not fully known.
Source (with guessed scores): http://www.gamefaqs.com/3ds/632930-luigis-mansion-dark-moon/faqs/66895
Mission     3* Requirement
A-1         1000
A-2         1700 – 2000
A-3         500 – 800
A-4         1300 – 1400
A-5         900 – 1000
A-Boss      -200
A-Bonus     2100 – 2400
B-1         1100 – 1400
B-2         1800 – 2200
B-3         500 – 600
B-4         1500 – 2000
B-5         1400
B-Boss      -600 – -200
B-Bonus     1400 – 1500
C-1         1900 – 2000
C-2         2700 – 2800
C-3         1200 – 1400
C-4         200 – 500
C-5         2900 – 3400
C-Boss      2100 – 2500
C-Bonus     2000 – 2300
D-1         2200
D-2         1400 – 1500
D-3         1000 – 1300
D-Boss      -100
D-Bonus     2000 – 2100
E-1         1900 – 2300
E-2         2500 – 2900
E-3         800 – 1600
E-4         -200
E-5         6400 – 7000
E-Boss      -400 – -200
E-Bonus     2100 – 2600
F-1         -200

