International Y (in widebodies at least) is actually very similar in Delta's fleet since they have not moved to 10-abreast Y. Relatively similar seat width and pitch across the fleet. My preference is 767 if only because it minimizes the number of middle seats - followed by 330 and then 777 for that reason.

Domestically, FlyerWX is right on with E-175 being the best. Wide seats and low density cabin (thanks to union contracts maxing out seat capacity at 76) means a high degree of comfort without the usual cons of RJs (don't have to gatecheck bags).

Overall, my preference is E-175 -> A320 family (with new seats), MD-90, 717 -> 757, 737, MD-88, 320 family with old seats -> non-Embraer regional jets.

This is based solely on seat quality as I don't use IFE. If you like IFE, avoid the MD and 717 families (although I think all have that Delta studio thing I've never used).