MMA Record: 23-10
UFC Record: 5-0
Thanks to Shaun M for this recommendation.
Let me start out by saying that Frank Shamrock is a true legend of the sport from when he was in his prime and is deserving of the accolades he has received. Shamrock would make a name for himself in Pancrase where he would become the Interim King of Pancrase and have a trilogy of fights with Bas Rutten.
He would go to a couple other promotions before coming to the UFC. In his debut for the UFC he would beat Kevin Jackson to become the first UFC middleweight champion (later renamed to light heavyweight championship). He would then defend the title four times, finishing his opponent in each one. The last of those defenses was against long-time Lion's Den rival Tito Ortiz. Ortiz would also prove to be the last legitimate fighter that Shamrock would beat in his career.
Then, in one of the most unbelievable career moves ever, Shamrock announced that he was retiring and relinquishing the title, citing lack of competition. Now this is where he lost all potential of being up there with other GOATs. Fedor Emelianenko hasn't stopped taking fights no matter the level of competition he is facing, same for Anderson Silva, GSP and Alistair Overeem. And with the UFC continuing to grow, especially at the weight class he was in, Shamrock was sure to have competition in coming months. Yet the ego of him saying there is no competition that can match him is absurd.
The retirement was short-lived, as he would return to another promotion a year later to beat tomato can Elvis Sinosic...you know, real competition. He would then fight Cesar Gracie after winning and vacating the WEC light heavyweight title. The Gracie fight was mired with controversy as it was Gracie's debut fight and he was 40 at the time of the fight. Even though many felt that he was an elite BJJ user, Shamrock knocked him out in 21 seconds.
Shamrock would then lose to Renzo Gracie by DQ after illegal knees to the head and hits to the back of the head. Again Shamrock would bounce back by beating "top competition" in Phil Baroni but would lose the last two fights of his career to Cung Le and then Nick Diaz.
There's a saying in fighting that goes "no matter how fast, strong, skilled or just plain good you are, there will always be someone out there who can beat you." Many fighters learn this throughout their career no matter if it's Fedor or GSP. If Shamrock wanted to leave MMA before becoming incapable of keeping himself on a winning streak then fine. But using lack of competition as an excuse is egotistical and lame on his part. Then for him to come back a year later to take on lesser opponents in between losses tarnishes his record of work and potential of greatness.