Sherdog member "TanMan" started a thread on sherdog.com about MMA's current win streaks:

Fedor Emelianenko (WAMMA heavyweight champion)
26 Fight Win Streak

Paulo Filho (WEC middlewieght champion)
16 Fight Win Streak

Miguel Torres (WEC bantamweight champion)
15 Fight Win Streak

Lyoto Machida (top contender to UFC light heavyweight title)
13 Fight Win Streak

Thiago Silva (top contender to UFC light heavyweight title)
13 Fight Win Streak

Urijah Faber (WEC featherweight champion)
13 Fight Win Streak

Anderson Silva (UFC middleweight champion)
8 Fight Win Streak

TanMan writes: Anyways I have Thiago going down next. IMO Machida has better stand up, ground game, and game planning on his side. If Faber ever moves up I have him also

Ted's Take: TanMan forgot UFC light heavyweight Rashad Evans, who is fighting Chuck Liddell at UFC 88 - Breakthrough on Sept. 6. I say Rashad goes down first before anyone else on this list. I guess you could argue that Evans (11-0-1) does have a slight blemish on his record (a draw with Tito Ortiz), but he has yet to be defeated.