I've been slowly working my through the various christmas special sandwichs. Here's a far from definitive list:

3. Starbucks - the stuffing is nice and they're usually quite moist. Decent bread.
2. M&S - usually do the best but...
1. Sainsbury's - decent amount of turkey, bit of veg and the stuffing really makes it.

Might have to re-evaluate this list.

Worst: Gregg's - ham?