No question that BC is the strongest. Being made out of a nonorganic substance makes it hard to deal with it beyond crushing it.
Dragons dont fair well vs. large beasts such as 'phants, because dragonfire is
potent as it melts stuff. If it simply shears off the outer layer, it is a wasted attack. Hydras are special as they have some resitance to fire due to there size, and there seven heads makes it hard to deal with for singualer fighters.
I have seen that Rocs are the weakest of the "true" megabeasts, because they realy have nothing going for them besides there flight and there size. Heck, it seems to me that there entire gimmick is that they can breed!
The semimegabeasts can best be sumed up as
Giant
Minotaur ( with gear.)
Etten
Minotaur ( just there natural stuff.)
Cyclops
The reason ettens are not higher is because a giant can simply whale on them and break them in horrable ways. A armored minotaur, on the other hand, can rampage thru your fort like a bull in a china shop.
1. BC can be defeated by dragons.
2. Hammers are useless against BC, use battle axes and other choppy weapons.
3. Dragons don't need to breath fire to kill, shields fair the worse of the fire. Doesn't mean it still can't bite you in half though.
4. Hydra's are cool.
5. Roc's can be fort - enders or simple amusement. Depends on the time they attack, but are usually easily dispatched.
6. Hydra, Dragons and pretty much every semi-megabeast can also breed.
7. Minatour don't come to fortresses in DF armed and armoured
8. Ettin get destroyed by pretty much everyone.
9. Bulls in china shops actually avoid the glass as much as possible. (mythbusters lol)