In no particular order, here are some of my favorite first lines of others' poems:
O hideous little bat, the size of snot
(Karl Shapiro, "The Fly")
When to the sessions of sweet silent thought
(William Shakespeare, Sonnet #30)
Because I could not stop for death
(Emily Dickinson)
We real cool. We
(Gwendolyn Brooks, "We Real Cool")
What happens to a dream deferred?
(Langston Hughes, "Harlem")
By the road to the contagious hospital
(William Carlos Williams, "Spring and All")
Glory be to God for dappled things
(Gerard Manley Hopkins, "God's Grandeur")
Is there any reason why a poem shouldn't
(Mark Halliday, "Functional Poem")
Body my house
my horse my hound
(two lines, I know; May Swenson, "Question")
i sing of Olaf glad and big
(e.e. cummings)
I hold my honey and I store my bread
(Gwendolyn Brooks, "my dreams, my works, must wait till after hell")
They fuck you up, your mum and dad
(Philip Larkin, "This Be the Verse")
When snow like sheep lay in the fold
(Geoffrey Hill, "In Memory of Jane Fraser")
I chopped down the house that you had been saving to live in next summer.
(Kenneth Koch, "Variations on a Theme by William Carlos Williams")
Nothing is plumb, level or square:
(Alan Dugan, "Love Song: I and Thou")
Lay your sleeping head, my love,
(W.H. Auden, "Lullaby")