When you use your phone as your alarm clock, you might think that you want to wake up to your favourite song. You do not. The first couple of days you'll enjoy it, but then the effect will reverse itself. It won't be "I wake up to my favourite song", it will be "sometimes iTunes plays my alarm". Every time your old favourite song comes on, you'll feel antsy or anxious.
If you switch to another song, the cycle will continue, leaving a trail of ruined songs in its wake, and you'll take months to recover your joy for these songs. (All of this also applies to ringtones.)
If you really want a personalised alarm, try something ephemeral, a kind of song that you wouldn't normally listen to for pleasure, but which brings back memories, such as a commercial jingle or video game theme. Or a song you wouldn't usually listen to, but has some novelty value for you.
But really, you should find an alarm that isn't musical. Sound effects are nice and hard to ruin; maybe play the sound of a zombie mob or a mysterious numbers station. The iOS clock app includes a great birdsong track hidden inside its "bedtime" feature. The birdsong starts quiet and rises in volume second by second. It's a nice way to wake up. And somehow it doesn't make me hate birds.