Perennial bulbs are bulbs "that will come back reliably for three to five years before diminishing". Most bulbs are, by definition, perennials. But not all will come back readily year after year in every setting. Soil, climatic and other conditions all play a part in determining which bulbs will be the best repeat performers in any given environment.
Naturalizing bulbs are "those bulbs that accommodate themselves fully to their new sites, feeling so at home that they multiply naturally, on their own, increasing in numbers year after year." Naturalized bulbs, once planted and established, will return year after year, often multiplying, to provide successive seasons of rewarding spring color.
Here's a sampling of top-performing bulbs that can be counted upon to perennialize -- and, if happy where you plant them, will most likely naturalize, too, to come back to bloom for years.