It's gotta be Frank Mir. Look at the accomplishments:
UFC Heavyweight Championship (One time)
Interim UFC Heavyweight Championship (One time)
Most wins in UFC Heavyweight division in history (14)
Most fights in UFC Heavyweight division in history (20)
Most finishes in UFC Heavyweight division in history (11)
Third most wins by submission in UFC history (8)
Most wins by submission in UFC Heavyweight division in history (8)
First and only toe hold finish in UFC history
Fastest submission in UFC Heavyweight division in history (46 seconds)
Defeated four different UFC Heavyweight Champions