Marciano was good to. A real fighter. There are reasons for the name Raging bull.
Raging Bull was attributed to Jake LaMotta. Yes Rocky was good. I see Tyson being mentioned with Ali so I do not see a wide breath of fighters being debated. My Dad's favorite sport was Boxing so I grew up getting to know a lot of fighters. Of course, because of my age, I would be drawn to the age of the 70's to 90's but I have seen some old flicks with Joe Lewis, and Sugar Ray Robinson. These two could easily be argued as the best. I loved watching Sugar Ray Leonard and his battles with the likes of Hagler, Hearns, & Duran.
Boxing has fallen out of favour for me as my liking of the UFC has grown. But I have to admit, I really like watching Manny Pacquiao fight as he is very exciting.
But all said and done, because of his Charisma, and his ability to adapt (considering he was not that powerfull), and what he did to allow good prize fighters to make good money (too much now for the likes of Floyd Mayweather) I vote Ali.