Royce Gracie - Obvious reasons.  He brought his style of MMA to the masses and you need to be well versed in his particular art to even be considered a fighter nowadays. 

Mark Coleman - Also known as the Grandfather of G&P.  His style of top position + donkey kong punches is still seen in nearly every fight.  He showed wrestlers there is a life after College and its called MMA. 

Ken Shamrock - His stint on WWE/F made many, otherwise unaware people, aware there was a real wrestling sport out there. 

Randy Couture - He showed people you didn't have to be an uneducated brute, such as Tank Abbot, to do well in this sport.  His common man persona with his superhuman skill and perseverance gave hopes to the normal guy.

Tito Ortiz - MMA's first bad boy.  The guy you loved to hate.  He made it easy for the UFC to start drafting story lines leading up to fights.  He brought a voice to fighters you virtually knew nothing about. 

Runners-UP

Chuck Liddell - On the list of "most recognizables" hair-do's, he's second only to MJ.

BJ Penn - Size doesn't matter.  It's how you use it.

Matt Hughes - (only for ppl in Iowa) Yes, you can become famous too!!