Which level 7 creature is the best, and why?

Probably, the Black Dragon is still the best – it can fly, has a lot of HP, and is immune to many things (even though soy won’t affect it, it definitely won’t be taken down by an explosion or anything like that), and it burns enemies within 2 squares – that’s three advantages (you can’t really protect ranged units against it), and it also has decent speed – that’s four.
Archangels will be a bit weaker, but if you’re talking about castles, then a Titan will definitely be more useful, as you don’t need to send it into direct combat; it can destroy everything on its own with each turn... The Black Dragon is ideal for all situations, while the others are better for specific purposes... :)