I think there were many, many more than you might at first think. over the 150 years , there were dozens & dozens of TOP fighters who could have won & held a world title, dozens!
It depends upon what you mean by "title". Burley held a strap. So did Sam Langford. If you mean a recognised, mainstream world championship though, it's Langford, McFarland and Burley.
3 strikes and your out! "uncrowned" would typically mean top guys who could've won a title but never got as much as a sniff at one!
Or fighters who did get a sniff at one but fought in a particularly strong era against very strong champions and or were considered unlucky in close fights for the title.
McFarland. He was arguably the best fighter from 135-160 and actually maintained an exceptional degree of consistency whilst fighting a lot of top names. After that I would say Langford, he very nearyl was lw and ww champion but there is little doubt that he was the best lhw for a number of years I'm talking from the Philly fight until Dillon began claiming to be champion. Then take your pick from murderers row who, coincidence or not, all annoyed their best years whilst their divisional champ was at war.
I knew there was a recent thread on this subject. This query was the greatest uncrowned in each division. http://www.boxingforum24.com/showthread.php?t=464099