I think it depends on what's causing you to feel this way. Break-up with a b/f or g/f? Your team lose? Lost your job? Someone ill? Loss of a family member? Loss of a pet? Accident? Hurt yourself? Crappy job with mean boss? Do poorly on a test? All of these can make you feel like crap, yet I don't think the same U2 songs will cover all emotions.
If you need uplifting, then I recommend uplifting soaring songs - the type that fill you with hope, like many from UF, JT and ATYCLB, like "Streets", TUF, and "Beautiful Day". If you need something more rocking just because you are feeling down today, then "Pride", "Desire", "Vertigo", "Gloria", "Elevation" and "I Will Follow" should blast you out of those blues. But if you need a feeling of comfort because of a break-up, songs like "One", "Acrobat", "A Man and a Woman", and "Ultraviolet" may work.
I often find, too, that hearing the live versions helps even more. Hearing the R&H era Bono soar on "With or Without You", listening to the ZOO TV Bono and Edge pound out "Mysterious Way", loving the playful sincerity on "Until the End of the World" from the Elevation U2, or tackling the world with the Vertigo U2 on "Love and Peace or Else" sure helps - especially with an appreciative crowd. And don't forget - Bono's operatic part on "Miss Sarajevo" always brings me chills and gets a crowd going.
I hope you feel better soon! And I'm sure there's some U2 for you somewhere!