Orks: RAMMING SPEED. YOURS IS THE PROW THAT WILL PEIRCE EVERYFUCKINGTHING.
Also, Heavy Gunz and hilarious ships.
No-one will ever out-ord you, not even Tau.
No-one will ever do less damage with Ordnance, not even Chaos.
Krakens are excellent, but have no models. I use modified Tyranid weapons.
Other than that, I haven't actually played with or against them much; I'll leave that to another anon. Same goes for Tau.
Dark Eldar: Allegedly flexible, wind up feeling like sneakier Imps with no broadsides, just L/F/R weapons.
Tau: Holy shit bombers. Maneuvering torpedoes. Overpriced, fragile battleships. Absolutely deadly in the front arc, shit broadsides; they play a lot like Imps with a deathwish.
Eldar: Escorts are excellent, and Pulse Lances are vile abominations. Die like whores to gunnery escorts, but their maneuverability makes them almost impossible for several fleets to catch (Orks, in particular), so just ball up and wait for someone to fuck up, then rape. Also, their ordnance is rare but appallingly powerful