Subway is the healthiest. Unless you're counting macros, what you eat isn't too important. Subway is the best because it allows for flexibility in exactly what you get. You'll probably get more fibre and protein in a Subway sandwich than you will anywhere else.
For weight loss the most important thing is calories which is what Subway is great at because you get a looooot of food for a few calories. E.g. A 6" sub is 200-450 calories where a Tesco Value sandwich is 350-500 calories. Also, the Tesco sandwich will lack a lot of macro/micronutrients whereas the sub won't because Subway use fresh(ish) veg, more protein, and less preservatives and other ****ty ingredients that Tesco use in their food.