For those who think of Longfellow as just a schlockmeister, I recommend starting with "The Cross of Snow" (his very private meditation on his second wife's violent death) and "The Slave's Dream." For those interested in great stories in verse, try the selections here from "Tales of a Wayside Inn" (yes, it's a darn shame that more was not included) and "Evangeline." For those interested in Americana, try "The Building of the Ship" and "The New England Tragedies" (the latter being verse dramas on religious persecution and the witchcraft hysteria in Massachusetts). And for those interested in formal experimentation, try "My Lost Youth" and "The Saga of King Olaf."
Like all Library of America editions, this is a beautifully printed book, with helpful notes and a chronology of the author's life. I just wish they had included more!