Ive to say with all my fleets (eldar, tau, sm/imp, dark eldar) and in all games ive played so far i had the biggest problems with chaos. (i didnt play mayn games (2) vs tyras though i guess they are pretty strong as well)
I think their "we dont have torpedoes but everything else twice as much as you"(range, lances, speed, cheap cruisers, cheap ordnance, assault boats, boarding modifier...) playstyle is a bit op :-D (dont take this too serious but the tendency comes close)