Which aircraft Delta is going to retire next?

At least some of previous generation A320 are supposed to be retired this year.

Then they have a bunch of 757 that are over 20 years old.

Delta already announced plans to retire its fleet of Boeing 777.  They are older -200 model and Delta only have 18 in the fleet and 11 are already parked.  Meantime Delta is taking delivery of the A350s, two of which Airbus handed over just last week. 

Delta will also most likely retire the Boeing 717.  With the Mad Dogs gone the 717 which shares many systems with the MD-80/90 became something of an orphan.  The A220 will eventually replace the 717s.

By the way, Delta has not retired its 767s.  It still has 75 in the fleet, 53 of which are temporarily parked.  I don't think even Delta knows at this point how many of them will be brought back and when.

The B77 is heading to the parking lot.

They will never retire the Peking Boulevard. 

Airbus A320-200. They average 24 years old - oldest in the fleet.