Do you mean just UFC or all of MMA?
Fedor Emelianenko is still undefeated so far and at least was considered the greatest fighter in the world. The only reason I say "was" is because he hasn't fought anyone from the top 10 in decent time frame. I'd still pick him over most fighter but it's hard to keep him ranked since he isn't fighting high caliber fighters at the moment.
Some notable undefeated fighters at the moment are:
Lyoto Machida
Rashad Evans
Fedor Emelianenko
As for the top five, it varies depending on the weight class we're talking about. However, these fighters would definitely be in the running for a top 5 pound for pound category.
Georges St. Pierre
Anderson Silva
Dan Henderson
Randy Couture
Quinton Jackson
BJ Penn
Gomi Takanori
Fedor Emelianenko
Hayato Sakurai
Antonio Nogueira
Kid Yamamoto(same as Fedor. Hasn't fought much recently.)
Mirko Filipovic(his spot here is a bit "iffy" as of late.)
That is list just off the top of my head. There may be some I am forgetting, but it's a good start if you're just looking to see who's who. I'll have to numerically rank them later. There's too much to conisder right now.