Of all the Delta credit card choices, it earns you the most MQMs, if that's what you mean by "best card for elite status". Not sure if this is possible for you, but if you can get both a Personal and a Business Reserve card, you can earn 60k MQMs/year if you spend $60k on each card. It supposedly still gives you a small advantage in your position on the upgrade list (after status and fare class, Reserve card holders are prioritized over non-Reserve card holders). And if you're going to be flying DL a lot, the Sky Club access is probably a useful benefit.

Honestly, once you've hit the MQM thresholds, I'd quit putting spend on the card unless it's for travel on DL where you're earning 2x. SPG points are more valuable and versatile (and can be transferred to DL at a better rate than you'd earn from spending on the DL card, with the 25% transfer bonus).