I top my 5,000 IU of vitamin D with 5 g of Omega 3, daily. There are many supplements with questionable benefits, but from all the research I have done, those two have a significant impact on one's health.
The omegas are a bit too much! And what Omega-3s exactly - you can't generalize as there's big difference.
3.25g EPA / 1.75g DHA. (For the record, that's quite a bit of calories, so I account for them in my daily caloric intake.)