I hate the Deimos class, those things are -fierce-. I always play FS2 on hard/insane and on those difficulty levels it's practically impossible to for a wing of bombers to survive an attack on a Deimos. In that one FS2 mission where you're ambushing NTF ships with the mjollnirs, my wing and I had no trouble attacking any of the other capships. But when that Deimos came in we were completely slaughtered, myself included. Even if you can get close enough to fire off your torpedoes it'll just -unload- on you with flak and you'll never make it out of its gun range alive.
The Aeolus class ships are pretty monsterous as well, but I voted Deimos because that utter ass kicking I received from one has left me mentally and emotionally neutered.