Songs in sharp keys (that is, keys with sharps, such as A or D) tend to sound brighter and happier. There's something about an F# that just rings. And it's not just me. The default ringtone on our home phone is in the key of F# minor, and the F# gets your attention. If I open my car door before I pull the keys out of the ignition, there's a beep, and it's an F# because that note will cut through and be heard. The beep on our microwave is an F# for the same reason. And of course there's the famous F# sung by our beloved James Labrie. It's not just that it's a high note, it's not just that he nails it. Part of it is the note itself. Something about a high F# just kicks your ass. A G# has similar properties, just not as pronounced. A lot of popular songs are in E, however, not just because of the G#, but because it's the key most guitarists start in. But historically, I'm sure there's a reason why guitars are tuned to E. It's a good key for a lot of songs, and Em is great for going the other way. Dropping that G# to a G really changes the tone of everything.
I recently played "Turn, Turn, Turn" by The Byrds, except they wanted to do it in C because the original key of D was too high for the singer. So that incredible, ringing F# on top was lost. C major is the first key anyone plays in when they take piano lessons because it's the easiest, but it's also the most boring. It has no personality. The song sounded flat -- not out of tune, just boring and lifeless -- in C.
Songs in flat keys (F, Bb, Ab) are mellow, and the more flats, the mellower. "Claire de Lune" by Debussey is in Db, and is super chill, yet rich and warm sounding. "La Pathetique" by Beethoven (actually Piano Sonata No. 8, second movement) is in Eb, but by voicing the chords open, it seems to broaden the sound, making it richer.
The wacky thing is that as you travel around the circle of fifths in either direction, the keys eventually meet and cross paths. "Fantasie-Impromptu" by Chopin is in C# minor and sounds like pure agitation and hyperactivity set to music. The middle section is in Db major and is the complete opposite, rich and mellow. But it's the same key. C# and Db. And the way he switches from minor to major and back again is brilliant. Chopin was a genius that way.
Anyway, my favorite key? It completely depends on the song and what you're trying to say. Some people say it doesn't matter. These people are not musicians, and I don't care if they play the guitar or the piano or the tuba. There's still a difference between a musician and someone who can play an instrument, and if you can't hear the difference between keys and what it does to the temperence of a song, you're missing something.