There are several characters to choose from to answer this question. George, Candy and Crooks each possess different, unique qualities of loneliness in the context of the story.
George is the only character of these three who has a friend, Lennie. Despite the presence of a companion, George remains isolated. Unlike the other migrant workers he encounters, George must look after Lennie and protect him. This burden compromises the single virtue presented by a migratory life for George - freedom.
Candy has no friends after the loss of his dog and he is not respected on the ranch. Though Candy takes up with George and Lennie, he sees the possibility of buying land with them disappear when he finds Curley's wife's body in the barn. Candy's loneliness is heightened by the fact that he is old and can no longer work and has litte hope for a positive change in the future.
Candy is the old, disabled ranch hand who is helpless to stop the shooting of his dog and who knows that he too will be banished when he is no longer useful.
Crooks is the most literally isolated character in the book, isolated by color and living conditions, but he is not a migrant like the others.
Crooks, the despairing old Negro stable worker, lives alone in the harness room, ostracized from the ranch hands.
In the permanent qualities of Crooks ostracism and in the permanance of his skin color, Crooks is quite different from George, Candy, and Slim who move from place to place, open to the possibility of change.