Some MMA candidates
Ronda Rousey
Anderson Silva
Georges St. Pierre
I was going to say, MMA has some great candidates.
Anderson Silva entered the UFC in 2006 18-4 and won 16 on the bounce over the next 6 years, usually in devastating fashion, including a dethroning and twice annihilating of Rich Franklin who, himself was a pretty dominany force with a 22-1-1 record when they first fought. He's lost twice to Chris Weidman, but no body thinks that's because Weidman is a better fighter (good as he is). Hopefully he's back with his recent Diaz win, but I think we're all braced for a retirement, if not now, then following one of his next couple of fights.
Georges St Pierre entered the UFC 5-0 and appart from his novice loss to Matt Hughes (in his prime) and a shock defeat by Matt Serra (legend) he's went 20-2 as a UFC Welterweight, including avenging both losses. Not having any of this Jonny Hendriks was robbed rubbish either. GSP clearly won 3 rounds of 5.
Talking of Matt Hughes, he was pretty dominant before GSP came along and spoilt the party. His 45-9 record has an 18 win streak, followed by two quick losses, then a 19 fight winning run broken only by a loss to BJ Penn smack bang in the middle. The game evolved and time is unforgiving, but 1998 - 2006, Matt Hughes was unarguably the greatest welterweight on the planet.
And what about Jose Aldo at Featherweight? 25-1, 15 straight in WEC/UFC, hasn't lost since 2005. Much as I love Conor McGregor, I just don't see a dethroning when they meet. Utterly dominant champion.
Not to forget Fedor Emilianenko too. Until Fabricio Werdum broke the spell in 2010 Fedor was 31-1-1 and the loss was a controversial 1st round 17 second doctors stoppage. Ignoring that and the NC against Big Nog due to an accidental clash of heads, Fedor won 31 straight against a murderers row on heavyweight/open weight killers. If only the UFC deal could have been done.
Ronda Rousey is 10-0, and that includes 9 1st round stoppages, and 8 armbar submissions. Cat Zingano's 9-0 streak is coming to an end saturday 28th Feb.
Jon Jones (sadly, cos he's a dick) has been utterly dominant, at least he was until Alex Gustaffson was robbed at UFC 165. He's gone 5 rounds in his last 3 fights and I think once he loses to Alex at the rematch this summer, much like with Fedor, the spell will break, and he'll look human again.
Finally, you have to mention Demetrious Johnson. 21-2-1, and his only loss in UFC was to Dominic Crus at Bantam Weight. As a Flyweight, he looks unbeatable for the foreseeable future.