Man made diamonds have come a long way since the plastic and glass junk of old. Most synthetic diamonds are hard to spot as fakes and many even cut glass as a real diamond will. Most are set in 14K gold or 10K to increase the real look and feel of the ring.
The best would be hard to determine depending on what you want to pay. There are some synthetic diamonds that can cost almost as much as a real diamond. Though I have seen some VERY nice synthetic man made diamonds on the market. I will give you a few links to the better man made diamonds and you can determine which ones are right for you and also fit your budget.
CARAT* Diamond Simulants
Moissanite Engagement Rings -- An affordable alternative to ...
Orleans Counterfeit Diamonds