It depends...........
There is no best ship ever...
The Perseus is very good at defending something like a big, like an installation, from enemy bombers...but it's dogfighting capabilies aren't as good.
The Mirmydon is the best ship for dogfights, install it a Prometheus S and two Subachs, some Harpoons and you are the king...
The HERC II, is a very good Assault fighter...Excellent at attacking small vessels.
The Erinyes, well, the Erinyes is a jack-of-all-trades, good at everything but excellent at nothing...Even so, it's my favourite ship.
and the Ares and the Mara-terran...They are extremely good, but i think they are also expensive for the GTVA. I mean, it would be awesome that every terran pilot had an Ares or a Mara-terran, but it would be unfeasable. It's better to have specialized squadron rather than all-purpose squadrons.