Probably your best option is the Amex Biz Rewards gold. $175 annual fee waived first year, and Membership Rewards points transferrable to Delta.

Generally the signup bonus requires spending $10K in about 4 months. (Sometimes varies from that.) I don't know what the generally available signup bonus is, but they run specials every few months, sometimes as long as a week, often as short as one day, with a 75K signup bonus. If possible I'd suggest waiting until their next bonus.

Chase Ink Bold is 25K for signing up and 25K after spending $10K in (I think) 3 months, but those points are not transferrable to DL, not sure if you have interest in that.