I think I've done a fair job of breaking things down, so let's take a look at the top five American-born fighters of all-time. (Sorry for spoiling you with the picture.)
5. B.J. Penn (16-7-2) - Penn is the most talented fighter on the list, but his career has been plagued by issues with motivation, as evidenced by losses to Frankie Edgar and Jens Pulver. Those losses aren't "bad," per se, but it's worth noting that Penn has never won more than four fights consecutively. What Penn has done, however, is win belts in two different divisions, including defending the lightweight strap three times after defeating Joe Stevenson for the vacant title in 2008. At 32 years of age, Penn is still relatively young, and with under 30 professional bouts, he hasn't become "fight old" either, despite entering his 11th year in the sport.
4. Randy Couture (19-11) - The first two-division champ in UFC history. Nine victories in UFC title fights. Winner of the UFC 13 title tournament. Wins over Chuck Liddell, Tito Ortiz, and Vitor Belfort. The only thing holding Couture back at the number four spot? Embarrassing losses to Enson Inoue, Mikhail Illoukhine, and Valentijn Overeem. Those losses are more palatable if Couture had a long run of dominance (see: Anderson Silva and Ryo Chonan/Daiju Takase, Georges St. Pierre and Matt Serra, or the guy at the top of the list), but, like Penn, Couture never won more than four fights consecutively. Unlike Penn, Couture took a solid wrestling game and an average overall skill set and turned it into one of the better careers in the sport.
3. Dan Henderson (28-8) - Like the two men before him, Henderson was a two-division champ, winning both the welter- and middleweight belts in Pride plus currently holding Strikeforce's light heavyweight strap. He also owns tournament title for UFC 17 and the 1999 Rings King of Kings. Henderson, in my mind, has only one questionable blemish -- his 2006 decision loss to Kazuo Misaki. His propensity for non-decisive decisions (he has eight non-unanimous decision wins) might give others pause as well. Otherwise you go through his losses -- the Nogueiras, Anderson Silva, Rampage -- and you see the biggest names in the sport. You'll find the same thing in his wins, though, including his huge legacy victory over Fedor Emelianenko last week.
2. Chuck Liddell (21-8) - Chuck was the last guy to defend the light heavyweight belt more than once. He held the belt for two years, and defended it four times after KOing Couture at UFC 52. In addition to his title reign, Chuck had two streaks of seven or more straight victories. He hit a sharp decline after losing the belt to Jackson at UFC 71, and a reduced Liddell still should have been Keith Jardine at UFC 76. In the middle of that decline, however, he looked like the Chuck of old in his legacy fight with Wanderlei Silva, one of the more underrated fights in history.
1. Matt Hughes (45-8) - Hughes, in my mind, is the obvious number one. Nine UFC title victories over two, long title reigns. Though losing both trilogies, he has victories over both B.J. Penn and Georges St. Pierre. His losses to Dennis Hallman are eyebrow-raisers, but the dominance more than makes up for it, and he had his own little Randy Couture moment when he submitted Ricardo Almeida at UFC 117. Hughes may no longer rightfully be the "Greatest Welterweight in UFC History" (GSP pretty much locked that title down when he started lapping the field at 170), but in my eyes, he's the greatest MMA fighter America has produced to date.