It's best to search all the relevant airline and credit card forums but:

- what city and country do you live in? I'm assuming the US given you have Delta miles.
- what airlines other than Delta might you use frequently?
- how important is status to you? Some of the Delta Amex cards give you MQMs towards status
- how important is lounge access? Some credit cards provide you lounge access
- do you want to redeem the miles for mainly domestic travel in your country or for international travel?

A site worth checking out is http://www.freefrequentflyermiles.com/index.htm