But any one of the three species could qualify as the latest arrival, depending on the year and where you see either Blackpoll, Conn. Warbler, or Canada Warbler in the spring.
Most important, note that typical annual returning times for songbirds (including wood-warblers such as those noted in the quiz) may vary from one year to the next. Predicting the abundance of night-time migrants has improved with the usage of radar technology, but knowing when warbler species arrive is an imprecise science.
Beyond spring arrival dates changing from one year to the next, vagaries associated with weather, wind direction (among other factors that vary from year to year during the migration season), always influence the timing of returning migrating songbirds. (To further qualify: note that Midwestern vs. East Coast wood-warbler species spring migration pathways and arrivals often vary, with fall migration offering a different set of migration and abundance dynamics for some warbler spp. in both of these areas.)
Which other wood-warbler species might rarely to infrequently be seen in northern climes during the non-breeding season? Common Yellowthroat, Pine Warbler, Yellow-breasted Chat, and Palm Warbler are the other leading species of interest when an "odd-ball" warbler is seen upon a winter landscape.