I would say Vaselin Topalov and I believe in the future that Magnus Carlsen will win the World Championship.

Korchnoi, Smyslov, Keres, Bronstein, Chigorin, Rubenstein

Topalov has won a title.

FIDE world title because he was the highest graded player but he hasn't won a world championship match before which is what we are talking about I think?

He did not win for being the "highest graded player."

He won the 2005 FIDE World Championship.

"In the face of criticism of the Knock-Out FIDE World Chess Championships held from 1998 to 2004, FIDE (the World Chess Federation) made changes for its World Chess Championship in 2005. It used normal (slow) time controls (unlike the 2002 and 2004 Knock-Out tournaments), and changed the format to an eight player, double round robin event, where every player plays every other player twice, once with each colour."

From Wikipedia: (original source link here: http://en.wikipedia.org/wiki/FIDE_World_Chess_Championship_2005)

As far as what you are "thinking about", I forgot to take my ESP pills today.

 The double round robin tournament that Topalov won to become FIDE world champion was San Luis 2005 and the tournament book by the same name is one of the best tournament books ever written and belongs in the library of every serious chess player.

Small point - Smyslov was world champion, though not for long. He won it from Botvinnik in 1957 and Botvinnik won it back in 1958. They played three matches altogether - Botvinnik retained his title in 1954 with a 12-12 draw.

For me Tchigorin, Tarrasch, Bogoljubov, Keres, Bronstein, Korchnoi (I also have to mention Flohr, Fine, and Larsen, and Geller)

Oh and Smyslov was a World Champion (great games between him and Botvinnik)

I think Keres and Bronstein really stand out though.

I forgot the great hypermodern thinkers that were overshadowed by Lasker and Capa, hard to be classified as great when going up against a couple of the most dominant players ever!

Lets not forget Karl Schlecter !  He only needed a draw in the last game to defeat the great E. Lasker for the world championship but lost the game.

Paul Keres

exerpt from Wikipedia: "He was one of the very few players who had a plus record against Capablanca. He also had plus records against World Champions Euwe and Tal, and equal records against Smyslov, Petrosian and Anatoly Karpov. In his long career, he played no fewer than ten world champions. He beat every world champion from Capablanca through Bobby Fischer (his two games with Karpov were drawn), making him the only player ever to beat nine undisputed world champions. Other notable grandmasters against whom he had plus records include Fine, Flohr, Viktor Korchnoi, Efim Geller, Savielly Tartakower, Mark Taimanov, Milan Vidmar, Svetozar Gligoric, Isaac Boleslavsky, Efim Bogoljubov and Bent Larsen."

Korchnoi,Aron Nimzowitsch,Paul Morphy and Akiba Rubenstein

N Short

Smyslov also won the title. 

 Your post is asking who the greatest player t never have won a world title.  Topolov has won a world title.  You didnt say anything about a "match" in the title or body of your thread.