My mom has this one, massive red candle that she reuses for all of our birthday cakes. I'm of the mind, however, that you should keep something a bit more clever on hand, for those times when you need to throw together an impromptu birthday celebration. No one can accuse you of forgetting their birthday if they're too busy laughing.
[$5, the Spoon Sisters]
Related: Happy Giant Birthday To You, Never Forget, Quick Tip of the Day: Eleventh-Hour Gift-Giving