This is a tricky one, because everyone on the Row is so nice. So maybe we need to look for our antagonist off the Row. In general, it seems that authority figures—like the "captain" who wants to kick Mack and the boys off his land or the watchman who tells the soldiers and their dates to beat it—might be voted Most Likely to Be Antagonists.
But you know what? Even the cops who end up partying with Doc and everyone else at the end don't seem particularly antagonistic. Maybe the real villains in Cannery Row are people who think property is more important than people—and, thankfully, they're most off-stage.