I'd say my top at the moment is probably the Falcon fire support, it really got my attention when I managed to solo those two onslaughts and three or thanks four smaller vessels without even taking a hit to my armor.
I must admit I'm quite fond of the Conquest thanks to the sheer weapon coverage it has, although the Apogee is quite nice thanks to that shield and the energy weaponry too.
Personally, I'm not overly impressed with what I've seen of the Onslaught. While it is unquestionably well armed, the lack of mobility combined with the limited front angle shield makes it a bit disappointingly easy to take out. I'll admit that this could be a result of the AI and the loadouts but even with a destroyer I've had little trouble taking out Onslaughts.
When we get the ability to refit ships ingame this will probably change.