For me it's definitely:
757 > 738/9 > a320
Most of this is driven by the fact that my upgrade chances transcon are better with the 757 and 738/9 (in that order) simply because there are more seats. I also prefer the 757 and a320 seats, however the a320 for 5+ hours can be long, especially if you get stuck in 1A/B/C/D (with no cutout) or a seat with the IFE box underneath. The 2L boarding with a much more exclusive bathroom is also nice in the 757.
The DTV on the 738/9 is a nice feature, especially when you're doing a transcon during major sporting events (i planned a transcon earlier this year specifically on one to watch the NFC / AFC championship games), but those seats for me are not even in the same ballpark as the 757 and a320 F seats.