Which level 7 creature is the best, and why?

Regarding Titans and CDs, I indicated what usually happened when I was testing monster strength (that was a long time ago; I would create two identical heroes in the editor—meaning class, skills, etc.—for different players, give each of them one of the creatures I was interested in, and play as both in hotseat). So, even though the Titan shoots, the CD manages to fly in before the shot and hits first + bonus damage to the Titan, and the Titan collapses in 2-3 turns, exactly one turn sooner than the CD (at least that's how it was in my experiments).

Added after 2 minutes
Oh, actually, you shouldn't compare them purely 1 on 1, because the result here is quite predictable; instead, try at least armies from a 1-week growth in a fully built city. My opinion is ownable/undisputable (IMHO).