Shredders under an FF, surrounded by tractor-turrets.
1500 Shredders/mark can do quite a bit of damage to any wave
Just keep them away from fighters (and similair ships that have a bonus vs shredders).
Weasels are fun when you encounter a lot of waves with frigates, though that doesnt tend to happen very frequent. When it does, however, even the counter-missile turret is awesome.
Best defence ever is, by far, the botnet golem. Not only does it waste 1k ships in seconds, it actually turns them to your side, so not only have you more or less killed 1k of ships, you just gained 1k ships in support as well.
Edit: Just to have it said, I
never pick a defensive startingship. (I dont see or instance shieldbearers as defensive units, they suck at defending your planet, just are awesome in preventing damage .. similair, but not the same.)